What Causes Hard Lump In The Knee With Swelling And Pain?

I am a male, 63 years old, and former pro tour tennis player ... 45 years ago (before scopes) I had major knee reconstructive surgery (ACL) and then 2 more surgical scopes 5 and 10 years later ... I m in excellent health for my age but my knee has this weird, large, bonelike bump that protrudes out every time I bend my knee past 30 degrees and it pops ... I ve grown used to the pain, swelling and limited motion but this lump seems to be getting bigger ... what is this? Thanks.