What Causes Bruise To Reoccur On Lower Back Of A Child?

My 23 month old daughter is very active and clumsy, so she has bruises on her legs. But tonight when I was bathing her, I found a bruise on her lower back, on her spine. This is the third time with in two or three months that I have seen this in the same exact spot. At first I just thought it was her being rambunctious, but now I'm worried. Should I make an appointment for Monday or should I stopped being paranoid. It's not a big bruise, just a small one; but in the same spot as last time?? What do you think?