Past Lump now reappeared

[edk1951]

I had a large elongated lump appear in my mid-back between the spine and left kidney just below the 12th rib the day after some heavy lifting. The lump itself was not painful, but deeper inside toward the kidney it felt as if tissue was being pinched. The lump stayed around for a week or so, began to diminish in size, and either went away or shrank to an unnoticeable size. Now it has returned in the very same place, not nearly as big. The location doesn't seem to match any muscles I've looked at online, but it looks like a big muscle spasm without pain. After reading scary information about soft tissue tumors, my questions are:
   Do cancerous lumps come and go, shrink and regrow?
   Does anyone have experience with a lump sticking up from their mid-back?
Thanks for any input.