kyle at 4.5 months old

well kyle started a new med for his acid reflux and well he stopped eating as much and didn't act like he usually do. he has always been a happy baby, and now he cry's.  it makes me feel bad because i dont know what to do to help him. he was 13lbs 10 oz at his doctor's appointment then i took him up to the hospital tonight because he only had 3 bottle all day, they sent us home to watch him, i feel like they should be able to do something for him,