I'm A 31 Year Old Female And I Have Been

I'm a 31 year old female and I have been noticing a bruise on the lower right side of my back that has been there for at least 6 months. It started out a circle shape the size of a pencil eraser and has since turned into much larger scattered bruise. I do have some pain and discomfort. I have not had any trauma to the area but do work a physically demanding job. What could this be?