I have a 7 year old German Shepard who has been very gasy so I changed his dog food about 3 weeks ago to Dimond for Large Breed Dogs. Since then he hasn't been as gasy but his stool is very loose, he seems to be a little constipated and he vomited twice last week in the house. The first time he vomited he threw up twice in one room and once in another room. Could this be a reaction to the new food? What should I feed him? Something for sensitive stomach?