What Causes A Bruised Feeling From Mid To Lower Back?

I have noticed that I have a bruised feeling from mid back to lower back when touched or applied pressure. I noticed it when I was laying on the floor with my 5 month old and my 3 year old straddled my back and sat down. I have not had any injury that I am aware of, I had an epidural when I had my daughter, also have had a recurring pain weakness in my left hip for about two and a half years..help?