My 4 week old keeps choking on milk all of a sudden? Please help

My 4 week old has just recently developed a problem where he acts like he's choking on milk whether it's directly from my breast or pumped and used in a slow flow nipple bottle. I have no idea what to do because he just starts gagging and stops drinking and then wants to try again a few seconds later and repeat. I'm going insane here. What do I do??