I have been playing guitar for about 2 years now. I am playing a set of .009s on a strat-style guitar. The action is as per my liking and not too high. Recently I have developed some pain in the first joint of my index finger on my left hand (fretting hand). This happens whenever I am barring two strings but not much when I am fretting single notes. There is no swelling/inflammation of the joint as of now. The rest of the fingers are completely fine. I think it's because of a rather long practice session I had once, but I have reduced my playing since then to about 1 hour per day now (in 2-3 sessions each day). The pain has persisted for about 4 days now and I am worried that I may have done some permanent damage. Any advice/suggestions are welcome. Thanks in advance!
See a doctor. Unless someone here is asking 20 questions because they can diagnose you don't listen. Your problem could be poor technique or it could be a real medical problem. As an example (ONLY AN EXAMPLE) I'll use myself. I was diagnosed with Psoriatic Arthritis a few years back. This type of arthritis shows up in the finger and toe joints, smallest ones first. Of course for me it was all the finger tips and all the time, not just when playing. In your case since you seem to correlate it with a specific action you might be lucky. But no one here can say, unless they are willing to prove they're an MD and then willing to risk their rep by diagnosing someone on a stack exchange site.
On the other end of the spectrum I don't think it's possible to assess your technique based on your description. It would help if you posted some pics, or a short video of the hand movement or position that induces the pain.
See a doctor
Post a pic of your hand while playing
This pain occurs right away as you press or it takes some time? Number of fret matters? (barring at 10-12th fret is easier than 1st) Try to barre using you arm as an experiment (you don't have to keep thumb and use less force), this could tell you whether muscles of your fingers are indeed a problem. Check your wrist position (keep it just a little bit slant, same your finger, so you won't barre with fleshy part).
You might want to give a guitar a break for a few days. Maybe your body need more time to adapt to that amount of practice.
It's probably because of your posture. Maintaing the right posture as it is always emphasized in music, is everything. If done correctly it should be possible to play barre chords without using your thumbs. Your index fingers should be straight when ever possible. But, if your posture is wrong, there will probably be twisting and torsion of your index finger, especially at the first joint. This means you also have to press harder to press that string causing further leverage on an already twisted finger leading to aggravated injury. For now take a few days rest from your guitar and take a hard look at your posture and see if that is causing the problem. Good luck. Hope this solves your problem. Bad habits will take time to correct but the effort will always be worth it.