Like Kaitlin says, voodoo bands are a good investment. Video tape your squat and watch your knees. There is a chance you are leading with your knees and loading the patellar tendon maximally before you descend. Along with using the band, make sure to smash your quads side to side over a softball or foam roller or have someone do it for you with their foot while you lay on your back. Start just above the patella work your way up.