My Son Was Born 3 Weeks Early. He Is Now

My son was born 3 weeks early. He is now 3 weeks old and constantly grunts, turns head side to side, and spits up following feedings. He also seems to get the hiccups every time he lies flat, and is extremely restless during the night when in his crib (we can see him turning side to side and hear him grunting on the video monitor). He also eats for short times but always acts like he is hungry. He is breastfed and is WNL for BM and voiding. Is the culprit acid reflux or what else could be going on?