Sounds a little bit like what I'm going through right now: hamstring tendinitis. Mine is in the insertion point of the ischium (my rear end), but you can get the same thing at the insertion point of the knee. It sounds familiar to mine, because mine developed as a minor ache resulting from running on snow and ice. Then I blew it out and made it really bad during a hard workout a few days later. I literally had to walk home, and I haven't ran since (been about four weeks).
Suggestions:
- Ice
- Heat
- Deep massage
- Ibuprofen (if you react well to it)
- Gentle stretching
- Chiropractic (might be caused by rotated hips)
- Physical Therapy