Well, I'm no expert (though I'm working on it), but it seems like that's something Pacman frogs do very often. Part of it is because they are ambush predators.
They don't want to waste energy hauling their fat butts around, so they have taken a sit-and-wait approach. They don't move or make a sound, like obese little ninjas, then when a cricket gets too close... BLAMMO! A sudden burst of energy! And if the frog succeeds in catching something to eat, it has wasted no effort to get it. Then, if some fatter, hungrier frog tries to get it, it will have the energy to slowly escape.
But, what to do if it's belly is full? Best approach would probably be to hide underground while it digests.
So, I'd say your digestion theory makes sense with what I have learned about these frogs.
I also know they go under to get cooler, so it may feel too warm. That's possible, but doubtful, because their natural habitat is warm and humid.
I've read that as long as you have the correct substrate and keep it sufficiently moist, it will take water in by absorbing it through the skin, and it will breathe so slowly that it can sort of suck oxygen through the mud...? Maybe someone can correct me on that.
But anyway, if he was healthy and eating a few days ago and his tank is up to snuff, I'd say he's just sleeping it off. How big is your frog and how often do you feed it pinkies?