5 week old has strange eating habits?

My 5 week old breastfed son has had very erratic eating habits. For the past few days, he has only been feeding for a couple minutes, projectile vomits several times, then has crying spells dotted with sleep. He does this all within an hour and then wants to eat again. If we feed him a bottle of expressed milk, he will eat 4 ounces but will gag and choke. I try taking it away but he sucks frantically and then screams when its not in his mouth. Sometimes when he nurses, he'll scream in pain and pull away, but cries if the nipple isn't placed in his mouth again. He is wetting and soiling his diapers like normal and it doesnt sound like a growth spurt. He has reflux, but we make sure he's upright all the time. We are exhausted and at our wit's end. What could be wrong and what can we do?