Bruises on bottom shell

by James
( PA, USA)

Ok I have had My pet turtle since 94' when I caught him accidentally with a Hook through his lid. I have taken care of him and got his eyes checked and they are fine now. I named him Fred but just after having him I found out he is acctually a she so now I call her fredina, lol. Anyway recently I noticed she has what look like 3 or 4 purple spots on the bottom of her shell and they are each on the intersections of the shell. The outside of each purple spot is sorta red, which makes me think of bruises. Im trying to crop the image's I took of her so that they fit and still show what Im concerned about. Hopefully it works, Thanks in advance if your able to help me out here.

Fred's shell is about 6" long and 4-5" wide and has a hieght of about 1 1/2", if that matters at all for this.