What Does A Spreading Bruise On Lower Back Indicate?

I have had an unexplained bruise on my lower back right side for a couple weeks. It is not painful and I have no lower back pain. We have been taking pictures of it to see if it is getting darker or spreading, and we marked around it with a sharpie. Today I checked it in the mirror and found that it has spread to the left side. It has been hard to find anything online about it.