First, you should really consult with a sports medicine doctor. That's the only way to truly diagnose and evaluate the best way to correct the specific problem that you're having.
That said, this problem is usually a result of a combination of over developed quads, as you suspect, and also a lack of flexibility in your hamstrings. Any hamstring stretch will most likely help, **_over time_**.
Try seated and standing toe touches, downward facing dog, and those seated one legged stretches where you reach for one foot while the other is folded in. You'll find plenty of other stretches if you do a search for "hamstring stretches."
Most importantly, don't push it. Gaining flexibility takes time more than effort. Pushing is actually counter productive. Just do the stretches **_gently_** and trust in time to do the real work for you.