More things to try: first try ibuprofen therapy, assuming it's tendinitis. Starting today, take 600mg ibuprofen every 6 hours, all day, whether it hurts or not. Time it so you take it when you go to bed and again when you get up. 6 hours doesn't need to be all that precise. Max of 2400 mg/day. Do that for 10 days straight. That short a time won't hurt you. I know people who've done that for 6 months, no problem. If that fixes it and it stays fixed for a at least a couple days after you stop the therapy, then you know what it is, and can take ibuprofen whenever it hurts. IME it would go away gradually over a period of a few months.

If that doesn't fix it, you can try glucosamine therapy, assuming it's a meniscus problem. Take 2000-3000mg of glucosamine sulfate/day in two doses. Also take 1000mg of MSM/day. Do that for a month. Some people it fixes right up, others no effect. If no effect you can discontinue if you want.

What quad exercises are you doing and how often?