Sounds like a colic to me. Call the vet and don't think twice about it. While the vet is on the way, keep the horse from rolling and keep it walking. Rolling will twist the gut, which is often irreparable.
It's a matter of opinion, but if I see something funny, I personally call the vet first and ask questions later. I keep the barn vet's number on my cell phone just in case, and twice it's kept me out of a lot of trouble.