Canker sores from oral sex?

Is it possible to give your partner canker sores (not cold sores— his are on the inside of his mouth, not the outside) a couple days after he goes down on you? It’s happened a few times now. I keep up with my vaginal health, and he’s been my only partner for about a year and a half now (and vice versa), so I’m pretty much in the clear for any STD’s. Is my pH just crazy? What would cause this?