My leaflet for prednisone does list diorrhea as a side effect, though I don't think it's a common one. Not vomiting though. It can cause reflux and heartburn too.
Are these symptoms you get ususally from Crohn's? Because in that case I think it's too early for the meds to be working yet, so you may still be getting Crohn's symptoms. If they're not "normal" for you (i.e. not a what you usually get from your disease, or more severe than what you'd normally get, which I think is what you're saying) it's probably something else - something you ate or flu, could be a lot of things.
Probably best to keep hydrated and check with a doctor if it carries on.
