A) Yes, I had (still somewhat have) tendonitis in my right thumb that was a result of two things: repetitive stress with less-than-optimal-technique and what's called an aberrant tendon in my thumb. Basically, an aberrant tendon means that the tendon in my thumb is attached slightly irregularly, which means that the tendon "pulls" at a slightly irregular angle. This condition makes me much more vulnerable to repetitive stress discomfort unless I'm careful about how I use my hand. When it first began bothering me, it was pretty painful. I didn't stop playing entirely, but I cut way, way down and I had to be very selective about it.
B) Well, the reasons above were the main causes, but I was also doing a ton of playing at the time. I was in my first year of grad school, and between my classes (most of which involved playing), ensembles, practicing, and gigs, I was playing for an average of 11-12 hours per day, sometimes less but sometimes more. That certainly didn't help at all, and it became a reall challenge to deal with an injury while trying to juggle all those responsibilties as well. I had to prioritize and drop certain things. Like I said, I had to be very selective.
C) The best thing I did was see an outstanding hand doctor. The first guy I saw was competant, but he didn't diagnose it right. He thought it was tendonitis, but then he later thought it was osteo-arthritis. After that I went to Kleinert and Kutz in Louisville, KY, which is one of the best hand care practices in the country. Dr. Kutz recognized it as an aberrant tendon, and he showed me specific stretches to do before I played so that the tendon would be flexible. He also put me on a regimen of vitamins (E, C, and B-6), and initially an anti-inflammatory medication as well. That treatment helped a ton. Beyond that I began working on technique and relaxation methods with a second private teacher who had also dealt previously with tendonitis, and working with her has made me much more conscious about how I use my hands, arms, and body (and lungs) while playing. The tendon still feels stiff and tight occasionally, but as long as I stay conscious of how I play, it'll be alright.
I must say that after all is said and done, the whole experience was really a blessing in disguise. The injury caused me to rework my faulty technique, and I play much better now than I ever have. I find myself much more relaxed and fluid at the piano. In a way, I'm glad I got injured, as strange as that sounds.
Peace,
Bri