Gasping?

I’ve had to start supplementing LO with formula. He’s so so so much happier and I’m thankful, but dammit with my body! Anyway, when he takes the bottle, he starts sucking normally, but within a few seconds he’s making sounds like he’s gasping, but is still sucking. I’m using slow flow nipples and have three different types of bottles and he does it with all of them. Actually when he nurses sometimes he’ll do it too. At first I thought it was he was sucking in air, but the more I’ve watched him I don’t think that’s it.

Thoughts??