Can cum turn into mucus?

I've been with my boyfriend for about 9 months coming up soon, and I usually swallow when I give him a bj but I find almost every single time an hour or so to a couple hours afterwards im coughing up white mucus and I'm just wondering if I shouldn't do it anymore? It just makes me feel kinda sick and he's reassuring and tells me I dont have to. I just can't find anything online that pertains to this topic, could it be a health concern for me or him or is this a normal thing? I'm just finding myself spitting it up almost every single time afterwards and the last time we tried I puked as soon as he came and I was still spitting up this mucus substance.