@lioness I don't know a lot, but I would think behind your knee it could be a synovial cyst since that's where you have synovial fluid. I'm glad you found something that helped.
So far, my thumb looks OK, it does not seem to be coming back. I have no idea what could have caused it since apparently often it is when a person has arthritis in their hands, and I do not. I only have had arthritis in my knees.
JK