A worldwide community photographing and learning about wildlife
Chrysemys picta belli
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.