Does your partner have any mental health issues? This is really common with men who are depressed or going through something psychological. Agree about the pot use issue as well. Hope it gets sorted out, has to be frustrating for you both.
Oh honey, calm down. This happens to men sometimes, and could have 100 different causes, and cheating isn't anywhere on the list.
He could be tired, he could have smoked too much weed or gotten a different strain that is affecting him differently, he could be stressed, he's getting older and things change, he could have low testosterone, a medical problem like diabetes, a prostate issue... the list goes on and on.
You also had sex 3 times in one day. That's a LOT. Is that normal for you?
Here is some more info on it, but don't worry yet. It was one day. This is only a concern (and a medical one, not a relationship concern) if it continues.
Don't accuse him of anything. Erectile dysfunction is not a sign of cheating by itself, and could be a sign of a medical problem, and I'm sure you'd hate to have accused him of something repeatedly if it turns out he has something wrong medically. Don't even bring it up again unless it keeps happening.
If he has a history of cheating, and that's why your mind immediately went there, address that separately from this. If he doesn't, and your mind still immediately went there, that's your issue to solve apart from him. Counseling can help. (I say that only with compassion, not criticism.)
It could be nothing (he was just tired or too high) or something more serious. Just be gentle until you find out.