3 Months Ago I Started Having Some Severe Knee Pain.

3 months ago I started having some severe knee pain. I couldn't fully extend my knee while walking for about a month. I went to see a doctor several times and after x-rays and an MRI he eventually just said to get a cortisone injection. Since that time I have been able to walk normally but occasionally pain will return. Some of the time this pain is accompanied by a small (1 inch) lump that can move around my knee. It is slightly painful to touch but mostly just painful to bend my knee when the lump is in a weird spot. If I move it around it eventually slides back into my knee somewhere and I can't find it anymore. What could this be? And what should I do about it? The MRI and x ray found some problems with arthritis but nothing with the lump.