Reflux babies

Mommas with reflux babies. What were the signs? My little one is a month old. He is breastfed and formula supplemented (Enfamil Gentlease). He usually eats 3-4 oz every 3-4 hours depending on how much he wants. I'm working hard to up my milk supply so I can get him off formula. He's spitting his milk up quite a bit this week. Sometimes right after feeding sometimes an hour or so later. This is our first week back to introducing breastmilk (antibiotics kept me from it) so I'm wondering if it could be my milk that's the issue?

Tonight he seemed really gassy in his sleep. I picked him up and burped him a few times. Then he pooped a lot. Yellow, very wet poop (big one). I'm just curious to know your experience with reflux. I'm not sure it's what he has, I'll be leaving a message for his Dr tomorrow to see if they think he should come in or not. It especially freaks me out when it sounds like he's gagging in his sleep. Just looking for others experience.