A sheetload is what you refer to when you've had a bowel movement in bed, probably while you're asleep. Eventually you wake up and soon realize something’s wrong considering the mess and smell. If you aren't sleeping alone, your bedmate will abruptly bring it to your attention. There is no denying what happened. If you don't have a mattress pad, you now have a nasty stain on the mattress. If you sleep bare-assed, the mess is even worse. Sometimes occurs when you're in bed and go to cut a fart that turns out to be a wet one.
Burritos and beer. That's all it took. My girlfriend got home late and climbed into bed and immediately alerted me about the sheetload I'd dumped without even knowing it. What a fucking mess!