holy crap... do you have any other information on what happened to have that occur? what all were you fighting? was it a camp? did you have any special units with you besides the sand golem? very odd
also, did you actually get that much XP? like, did you gain a ton of levels after the fight? or was it just a display error?
-edit: noticed that a couple thugs have levels of -1... that's probably the issue, do you remember where you encountered them? were they on a lair or something?