I began running when i felt a pain under my knee cap. been resting 6 weeks due to back fracture. what is it?

Weak quadreceps: When you were not running the quadreceps muscle got weaker and is now pulling the patella (knee cap ) towards the outside of the patelar grove, a dent in the fefur that the patela moves in. To strengthen the quads use moderate weight and a short no more than 20 degree range of motion, lift the knee, tighten and hold then relax and repeat, this will help get back quad alighnment.