If there's an internal blockage perhaps, what he ate can be spoiling inside his body. Ever see a dead creature? As they spoil, bacteria makes them bloat up...and it sounds like that's what is happening with your snake. Cannot tell you why, though...I hope your vet can. Don't "panic" but DO get some help for your snake...this is NOT normal. (It's very unlikely to be IBD though, btw.) There might also have been something wrong with the feeder you gave him...try a different source in the future. (if the rodent ate something bad & it was still undigested, that might make it so your snake cannot digest either)
Another thing (& I HOPE I'm wrong!) that can cause a snake to bloat up is serious & life-threatening but not contagious: gastric torsion. Some kinds of animals are more prone to this than others, but even where it's rare (like in snakes) a given individual might have an imperfect digestive tract that lends itself to this predicament. If this is suspected, you must feed only small meals (and NO handling until digestion is complete), and that's IF your snake survives this. (often surgery is needed to untangle their twisted GI tract)
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