Flatus vaginalis is air coming from the vagina during or after intercourse. In popular speech it is called a vaginal fart.
Air is pumped into the vagina by the penis moving in and out. In some positions this happens quite easily, like doggy style.
This air has to escape, making it sounds as if the girl is farting, but it comes from her vagina and is odourless.
Of course this can be embarrassing, but it is no big deal when you are both able to laugh about it. Try once - for fun - to produce as many farts as possible during intercourse. After you have tried this it will only be amusing if it ever happens again.

Ways to avoid it