What Causes Pain And Swelling Behind Knee?

I ve noticed I ampainful behind my knee and there is swelling and it is affecting my stride and ability to completely flex while walking it sometimes goes almost away but seems to flare up .i am worried it may be a blood clot or bone cancer ,also my leg from the knee down feels strange and some. Tingling. Pinched nerve maybe? It does get aggravated more when I kneel down