Yes, we have come accross a lot of question like yours where people has some problem and blame it to masturbation. Somesays losing hair, some says gaining more hair, is it because of masturbation? Some says Losing eye sight, some says keep forgetting, and the blame is on masturbation.
Comeon, try to find the real reason why you are having pain in your knee. There can be so many different reasons, try to find the reason without blaming masturbation.
You might be a sports man and got hurt in early age, later it can relapse. The other reason is lack of calcium, taking medication like calcium lactate or multi vitamin can be the solution. You also can have rheumatic pain, knee are supposed to pain, there are medicine for that. And again, it might be sometime you took a bad knee position on masturbation, that also can be the reason.
I am not going to more serious one, as we donot discuss about medical issues here. But if that is the reason, you need immediate medical attention, if teh delay happens, this will go for worse. Its better not to scare off here, rather hear it from the arthopedics after necessary tests.
Stay well and get well soon and ont masturbation on everything happens in life.