On the back of my left leg there is a lump about the size of a peach pit at times. It is very tender, afte I have been sitting or laying down and get up it is difficult to walk for a minute or so. I just noticed the lump about three weeks ago. I think it has been there because of the pain after getting up. The pain has been going on for about four months. The lump seems to be on my achillies tendon or on and partially beside it. It get larger at times and then goes down but no away. It is even with my ankles, but on the back of the leg. What is this and what is done for it.