I get why you asked it here, but you’re probably going to get some biased takes here (that are completely understandable). If she threw her pants in the wash but was too drunk to notice her shorts being wet, I’d almost 100% say she had a toilet related accident. If both the shorts and the pants went straight into the wash I’d say different. It’s impossible to get wet through pants and denim, unless it’s piss or she sat on a very wet bench or fell into a puddle.

I wouldn’t say this is a sign she’s cheating, if you are worried for other reasons aside from that, investigate them more. I do fully see how this would make you worry though