Dog has upset stomach?

i have a 1.5 year old,52lb neutered boxer male, and he has been licking his lips as if he is about to vomit, so i let him outside to see if he would vomit and instead he was gobbling up grass, i gave him a teaspoon of pepto bismol and he now seems better. This is the second time this week that he has gone through this and I dont know why. He has been on Orijen adult dog food since he turned 1 and gets 1 meal in the morning and another at night, he was fine but then all of a sudden he looked like he needed to vomit. his gums were pink, he was still responding to commands and he ate pieces of carrot and apple as treats fine...what can this be?