Re: Tendonitis, OUCH!
I've got it in both hands, had it for a little over a year. Take it from me: there is no cure. I've not played guitar for almost two whole months, and there's been virtually no improvement. Fancy wrist braces that I got from the physio haven't helped either, and I'm still on the waiting list for proper physio treatment. I may never be able to go back to playing guitar again. I can't tell you how depressing that thought is.
Bottom line: if you think you have tendonitis, STOP whatever you are doing IMMEDIATELY, get a doctor's appointment, take anti-inflammatory medicine (ie ibruprofen, though I think aspirin is also anti-inflammatory?) and DO NOT go back to whatever has caused the problem in the first place until you've seen a qualified medical professional.