I would probably keep them separated for the rest of the day following the vet visit. Do try some feliway to see if it will help "diffuse" the situation.

This is not an uncommon problem at all, but most clients I've talked to with the issue say that it usually resolves within a day or two. So try to keep them separated if you can't supervise them... kind of like when you're introducing a new cat to the house hold.