western painted turtle

Definitely not shell rot, Karen is right. I have one of these as a pet (though a little bigger than this spotting!) and his shell is a similar pattern underneath.

That's not shell rot. You'd see a degradation of the shell. Having worked in the pet industry as a young adult(for a few decades) you see these guys fairly frequently(and the resulting diseases. Yellow eared sliders have a yellow belly. There is, if I remember correctly, one subspecies where the young are more brightly colored, but they fade as adults. This is a painted turtle.

May be Yellow eared sliders...

Karen: Are the shell usually red like that or is that shell rot?

maybe it's yellow eared?

doesn't have a red ear

It looks like a red-eared slider with shell rot...

Like a Red eared one. http://en.wikipedia.org/wiki/Red-eared_slider