I know EXACTLY where you're coming from. Last month I had an ear infection and went to the doctor. I'm allergic to penicillin, so she gave me Levaquin. I read the pharmacist's insert to get an idea of what kind of side effects it could have (I always do that - I tend to be really sensitive to medications. For example, I've got plantar fasciitis in both feet and in the early stages the podiatrist put me on an anti-inflammatory. After the first day, I had horrible heartburn. He took me off that and prescribed Celebrex, which was supposed to be less likely to cause that side effect....um, wrong. I use aspirin for pain - sometimes Tylenol, but that doesn't seem to do as well as plain old aspirin) and thought there wasn't anything that looked too bad in comparison to the ear infection. After 3 days I took myself off of it (4 days before I was supposed to see her again). The first thing I noticed was my left elbow being very tender. I thought it was because after hubby put the eardrops in my right ear, I'd just fallen asleep on that arm and slept wrong so I didnt give it a lot of thought. 2 days after that, my forearms were so sore I couldn't lift anything. That's when I got the sheet out again, and sure enough, one of the "rare" side effects is tendinitis or a possible rupture of a tendon. I stopped the medication right then. When I went back to see her, I told her what had happened, and she said I was only the second person in 8 years that had complained of that.
My forearms are fine now, but the elbow still bothers me, and I've noticed my knees are giving me more problems than they used to if I sit for any prolonged periods (like to knit). I'm still thinking time will work that out, too.
Hang in there, it will get better. It just takes time to get the stuff worked out of your system. The best advice I can give is to just rest the muscles involved as much as possible, and if it really doesn't start to get better, get in touch with the doc and see if there's someone else you can see about the tendinitis. You definitely don't want to be messing around with permanently injuring them!!