Why do I feel burning during sex? (no condoms/other symptoms)

I've been experiencing burning during sex. Its not really painful, but it is annoying. It's happened a few times during this past month. It always happens during round 2+. It never happens when we first have sex for the night. I'm wet, so he's not going in raw or anything. We usually have sex every two days. We don't use condoms/I have the IUD.

I've looked this up on the internet, but it's always associated with other symptoms or having a UTI/yeast infection. The thing is, I don't have these things. Everything is normal down there.

So why is my vagina burning?