Sounds like Patella tendonitis...It can happen at anytime even if your knee felt healthy up until that point. It can last several days, weeks and even years. Most of the time meds, Physical therapy don't really help all that much. Rest it for a few weeks and see what happens. It sucks for sure..I had the same issue after squatting heavy and its been about 10 months...it's better but still aches/hurts most of the time.