What is a lump on the back left side just below pelvis but not quite buttocks about size of quarter that does hurt when pressed on?

It's possible this may be:

1. Kidney stones: Kidney stones often cause pain in the back and side that can range from mild to severe. The pain typically starts suddenly and may come and go in waves.

2. Muscle strain or spasms: A muscle strain or spasm in the back can cause a lump or knot-like feeling. This usually goes along with pain or tenderness of the muscle.

3. Herniated disk: A herniated disk is a condition in which the soft inner material of a spinal disk leaks out. This can press on nerves and cause pain, numbness, or weakness in the lower back or leg.

It's recommended that you see a doctor for an evaluation as these conditions requires different courses of action.