The red marks could be the beginnings of a thermal burn which would be caused by temps being too high. The best thing you could do here is go full disclosure and show us the setup at the very least, we might be able to see at a glance anything that looks really wrong.

Belly burn (assuming it is that) screams to me non-thermostatted heatmat. A better photo of the belly might be in order as well, to identify if it is that.

Best of luck and just remember we're not here to chastise you, we're posting because we're trying to give you the best possible advice for the good of the animal; it is nothing personal.