I had a surgery on my knee about two years ago. I had half of my cartilage removed, and my acl reconstructed using some of my hamstring tissue after a sports injury . I have been working my leg at the gym as I find since the surgery and even physiotherapy , it is always slightly weaker than my other leg. A bruise, that I believe just came out of no where has appeared on my knee scar. It is tender to the touch, and my knee is just a little ache out of the norm. But, nothing at all bad. It really only hurts when I touch it. But my main concern is not the bruise, but this little ball I can feel under it, now. Again, though, only when I touch and poke at it is it only slightly painful. Thank you.