My dog just yelped loud, could it be new food?

I just finished swithching my dog over to blue buffalo large breed fish and oatmeal over the course of a month, from iambs. He has been eating only the new food for 4 days now. He was laying in front of me after playing for a bit and out of nowhere yelped three times in a row really loud! He got up but I had him lay back down and felt his stomach and it was rumbling so I wonder if he had gas or something? This was 2 hours after eating his dinner. Should I back off of the new food or just see? He hasn't had diarrhea or vomiting though he seems to be pooping more often like three or four times a day instead of two.