I'd have to see you play but even then, what you are doing wrong might be invisible. You should find a teacher who works with injured pianists and fix what may be wrong with your technique. I am just going to ramble some repetitive thoughts for you to ponder BUT, If I were you I personally wouldn't change anything about my technique without working with a knowledgeable teacher because you could make things worse.
It is the weight of the arm which is the dominant force in depressing keys. Many people play from what they think are their finger muscles (which don't exist). Our flexor muscles in the forearm are what move our fingers. However, when we use those muscles we often strain the long flexor tendons between the bone and muscle. It becomes a tug of war between bone and muscle and the tendon is caught between. The fulcrum of the elbow is a very important first step in providing up/down to facilitate gravity. Notice if you play a chord from the elbow using up/down, there is very little effort coming from the fingers because you are using your flexors minimally. Also, the flexors fatigue quickly and easily. As you learned in HS physics, every motion has an equal and opposite motion. Kick a ball, swing a bat, slap a face, poke an eye, swat a fly, throw a rock, swing an axe . . . Why not the piano? It is because we try to play like people who look effortless but in reality, there is a lot of invisible movement going on. Plus, we tend to look at their hands and not their elbows where all the real work is going on.
The second thing you may be doing wrong is pressing into the key bed. Once you play to the point of sound (that little bump you feel if you depress the key very slowly) there is no reason to go any further. Pressing into the key bed produces no more tone so why press? It only strains the long flexor tendons.
Another thing could be you are not using your shoulder and elbow to place the hand where it needs to be. Not doing so can cause ulnar and radial deviation or, a twist in the wrist. This hinders technique and any time technique doesn't work, we often employ more of the wrong muscles to compensate and a downward spiral ensues.
Along with the above is aligning the forearm behind the finger being used. Your two and three fingers feel strong because the forearm is right behind those fingers, they are a straight line. The four and five are at angles and many teachers tell the student they need strength and endurance or more practice but what they really need is to learn how to make subtle adjustments to the alignment of the forearm behind those fingers and the four and five will feel just as strong and coordinate as the others.
In/out may be missing. Many pianists curl their fingers in an attempt to equalize them. This curling, you guessed it, strains the tendons. Tendons are pulleys and rubber bands designed to move bones, not do the work of the muscles. When teachers tell students to relax their cramping muscles, they do but then strain the tendons. The solution is . . well, the first step is to get gravity into your playing. Gravity is effortless. Your up muscles are much MUCH stronger than your down muscles because we spend our lives fighting gravity (bicep/tricep, quads/hammies). Use the up to harness gravity to do the work. Some pianists think some pianos are stiff, that is because they are trying to use brute force to play rather than physics.
You may be using two muscles to move one bone in two directions at the same time even though you are trying to do only one thing. The most common example of these vector forces is using the abductors and flexors simultaneously. Or, isolating a thumb or pinky. Abductors are very weak and sluggish. To engage them creates instant tension. Go ahead, spread your fingers out. You can immediately feel them want to adduct. Too many of us begin our playing with this tension before we even touch the keys. Have you ever spread out your fingers over the keys then miss the first note? It is because your abductors are pulling the hand in several directions at the same time when we really want to play straight down.
As for the thumb, most of us are taught to cross it under the palm. This locks up the extensor tendons and also grinds with the forefinger tendon as they intersect. It would be better to play the thumb from the pronator muscle which is effortless. Combine the pronator with gravity, playing to the point of sound and you won't feel your hands at all. The pronator and supinator muscle, BTW, are indefatigable. They can go on and on and on with little fatigue. The thumb's weakest muscle is its abductor, remember, abductors tense immediately. Guess which muscle most pianists use to play the thumb down? The abductor.
I would urge you NOT TO TRY ANY OF THIS AT HOME. You need a teacher. Trying to apply any of this could make you worse. There are about ten proper movements needed for playing and about ten we should never do. If you correct one and not another, you could create new problems. The art of effortless playing is a combination of several proper movements which work with one another at the same time. Like one of those crane type lamps. You move the lamp head and several levers or arms move from the original movement. Nothing can exist in isolation.
Lay your arm on the table and pretend you are casting a fishing pole from your wrist. You might feel tension but the point is, you won't cast very far. Now cast from the elbow. You'll go further and it will be more effortless but still not optimum. Now cast from the shoulder. Much better but still . . . Now stand up and cast from you calves, hips, abs, shoulder, elbow, wrist and fingers . . . Everything works together for optimum efficiency and power. Playing the piano is the same.
Any time you feel pain, fatigue, cramps or tension, it only means you are doing something wrong or something is missing. You need to have a teacher do some detective work to find out what it is.
Again, this is not a jump off point for you. Find that teacher. Start over. Bad habits are very difficult to eradicate once hard wired into our brain. Technique is all in the brain, BTW. It is knowledge, physics and biology. Not some hocus pocus, practice makes perfect, brute force or talent myth.
Everything I know comes from being paralyzed for two years then working closely with a teacher to fix all my bad habits. I am still working on it. Sometimes you just have to hit rock bottom. I was "lucky." I was in so much pain the only movement I COULD do was proper movement. I couldn't even pick up a piece of paper but my teacher hand me playing single notes in seconds. I was pain free in less than an hour BUT it took me several months to heal. I actually don't beleive I am healed but, I no longer use the wrong muscles. Much.
