The feeling you get instantly after ejaculation when you have had sex with someone you shouldn't have. The pure natural and unstoppable instinct of emptying your sack took over all rational thought then instantly returns the split second you orgasm
similar to instaregret
Dave: - "mate i was hanging out the back of this well fit bird last night coz the missis was away with mates... but fuck me i got instaguilt the moment i blew my load"
Tony:- "lol, i know what you mean... its like the worst feeling ever"