Hit it off with a girl but turns out her downstairs smells *extremely* bad - (how) do I tell her?

The first two dates lasted hours and went really well, then on the third date we slept together but she emanated the worst smell from down there that I have ever experienced - to the point that it filled the room, made me feel sick and has completely turned me off her sexually. It wasn't clear whether she was aware of this terrible smell or not, we never acknowledged it.

Do I tell her straight up about the smell or do I make up an excuse for why I'm no longer interested? She is a nice girl, but I can't see past this even if it was a temporary thing.