Slightly movable lump, low back, left of spine. feel more when bend over. possibilities? past hodgkins lymphoma and have fibromyalgia w/back pain.

Bursa: It varies, and with your history I sure would recommend you being seen for it. That being said, there is a bursa that normally overlies each of the sacroiliac joints and it would likely be in the area you are describing. They can become enlarged when they become inflamed or when the SI joint is abnormal. It is not uncommon for patients to present to my office with that exact question.