It definitely could be from the diarrhea, it could also be if he was doing a lot of licking (due to the diarrhea) and now he has a yeast infection. It could also be that his anal glands are full and irritated. If you can-have the vet look at it again. If not, what I would do (and I'm no professional) is make a solution of 50/50 water and vinegar and wet a paper towel with it and pat his bum a couple times a day to see if it helps. And then if it didn't get better I'd take him in. I know, it can be a pain in the ass to keep going back to the vet, but sometimes you gotta.
We had a bought of butt bleeding with our boy Trooper once-way back when...and we took him in because we'd left him loose in our bedroom (he was previously crated) and he ate everything! Tore up a leather belt, ruined a bra, chewed on the window sill, pulled our sheets back and tore up the egg crate padding we had on the bed. We took him in for x-rays because we couldn't find the buckle to the belt. We were told he was full of poop but no buckle. He ended up being diagnosed with colitis (likely from all the weird chocolate he ate). So maybe your pup got into something that just didn't sit will with him and it caused some colitis. *shrugs*
We ended up finding the belt buckle in a pile of dirty clothes that were by the hamper but not in it...as if he burried it there.