Lol whyyyy does he stop me now

My boyfriend has ALWAYS loved my blowjobs the most, it has always been his favourite part of sex, since being heavily pregnant and suffering bad heartburn it was getting a little harder for a while. As the acid was coming up my throat and a few times I was almost sick on him. Lol. I'm now 37 weeks. But I've noticed this little while now that when I go down on him, he stops me after a few mins. 🤔..... I'm starting to think he don't enjoy it anymore. I did ask and he laughed and agreed yes hes been stopping me, but he wouldn't say why, he just kept kind of laughing. I said have I lost my talent lol he was like NOOOOOO, just got a little use to stopping you as I Was feeling bad about your heartburn. I told him I haven't complained and I'm ok now it's settled. But he was acting all shy and funny. Lmao. Whyyyyy is he suddenly stopping me I dont get it.