I hyperextended my knee a year & didn't break anything, but now after i run the muscle in the back of my knee hurts when i straighten it.

You may have...: Strained your biceps femoris, or the large posterior muscle of the thigh when you hyperextended your knee. The can be a devil to heal. I would cut back on running and perhaps cycle for a while. If your problem presists, see an orthopedist!

Get it checked.: You may have a muscle strain/tear in that area, but it may be something more, like a baker's cyst. See a non-surgical sports medicine doctor for evaluation.