What could a painful lump below the back of the knee be?

Answer:
Turns out in my case it is a bone tumor. Just learned this from an MRI. Hopefully it is the benign giant cell tumor that the radiologist thinks it is. I am keeping my fingers crossed that it is. This lump was not visible, but could easily be felt when you pushed on the spot. My major pain was in my big toe. Turns out these tumors can cause pain in joints that aren't even affected. If you feel any lumps that seem unusual, I'd recommend you get them checked out. I wish I'd gotten mine checked out a year and a half ago when the pain started. I didn't have a lump that I am aware of at that point, but I had intense pain when anything put too much pressure on the area.
First answer by Rhysanna. Last edit by Rhysanna. Contributor trust: 0 [recommend contributor recommended]. Question popularity: 3 [recommend question].