I've been playing / teaching the piano for years with no problem but recently started a new piece; two days in and I have a swollen and painful injury just above my wrist, which I can feel across the back of my hand and further up my arm and it's definitely connected to the 2nd finger. I need to remedy whatever I'm doing to make this happen so any suggestions? It's getting iced but in the long term I think I need to know what has caused it, plus I'd like to pass this information on to pupils.
Find a really good physical therapist, preferably one who works regularly with musicians. I had tough issues 18 months ago with (what I thought was) tendonitis. I play sax, clarinet and guitar. It was severe enough to cause me to take a break from playing.
I initially saw one specialist who thought it was carpal tunnel, and recommended various exercises. However this diagnosis turned out to be incorrect - the issue resurfaced recently when I tried to start playing regularly, now in both hands. I found another therapist, and the problem turned out to be the weight of the sax strap on the vertebrae messing with the nerve, combined with various postural bad habits. After several sessions I have a new sax strap, am doing various exercises for my upper back, and am much more attentive to posture (also while driving, typing and so on). I'm playing more and more easily as a result (and stand better too).
Your particular problem is likely different, and noone can tell you what it is without a lot of experience and seeing you in person. But get it checked - its probably less of a big deal than it seems, once you find and rectify the cause. These things can come on suddenly and severely, and are a sign that you need to change something.