I Have Been Having A Lot Of Knee Pain And

I have been having a lot of knee pain and have noticed a lump forming on the outside of my knee kind of under my knee cap. It is only visible when my knee is bent. The more physical activity I do the lump enlarges but swells back down once activities slow down. I've already had an extra and an MRI the doctors don't know what's wrong with it. They had thought it was osgoodschlatters syndrome but it wasn't. Please help! I would be so thankful!