Pain on base of patella (not the tendon). What can cause this?

Tendonitis usually. The attachment of the tendons or ligaments to bone can get inflamed - insertional tendonitis, assuming no fractures on xrays. Usually this results from somewhat tight quadriceps (thigh muscles), and a course of stretching with antiinflammatories helps. If this is not an isolated site of the inflammation, with other places in the body inflamed as well, a rheumatological workup would be in order.