He totally looks bloated to me too...since you know he's not constipated, you should get him to a qualified herp vet a.s.a.p. to see what's going on...does he feel
squishy in that area? Could be fluid...?
How much are you feeding him (every 10 days)? I can't imagine you are feeding him enough to do that...I think something else is going on.
What did his stool look like? hard or soft? If you can, "next time", save it for vet to check...it has to be fresh (& probably not underwater but you can ask?).