Best advice, I don't want to be alarmist either, I still struggle with when to call, but with a nursing mare, I'd be on the phone already describing symptoms. The vet may have other things he'd want you to check, and may say to watch longer, but I'd suspect he'd try to come right out.
I think I will call, just to be on the safe side. I'm going down to take the grain away and make my call.