After both knees being really good for a while a misplaced foot getting in the car and now my left one is bugging me. Foam rolling the quad seems to help, so I've been trying to be disciplined with doing it daily.
4/17 - Pretty sure it's quadriceps tendonitis. Slowly making progress in getting it better with a lot of stretching and icing after workouts.
4/24 - Starting to feel better, lots of stretching, and running actually seems to help a bit.
6/8/15 - Acute pain in the left knee seems to be under control. I think what I have now is old knees. Lots and lots of quad and ITB stretching was the key.