ironically i had a similar thing happen to a carpet and it went away... i assumed it was some sort of blockage above the stomach, causing the prey item to not get to where it needs to go. it was there for a couple weeks... anyway, this was quite some time ago and he hasn't had any problems since it went away.
dr del, i agree, too high for the 'stomach', yet too low for an enlarged heart...
good luck! a quick xray would definitively tell ya though... judging by whether he's otherwise 'okay' doesn't really work for reptiles though - CNS is completely different than mammals.