How do you tell a girl that her vagina smells?

I've been seeing this girl for a while, not dating, just fuckbuddys, She is a very clean person, don't get me wrong, but her vagina emits this highly unpleasant odour that you can smell from upstairs - all the way to downstairs, I've seen her clean herself and as soon as we go lay down and start to get freaky, she starts smelling again, so I'm thinking it's her personal lubricant smelling. I don't know how to tell her about it without her getting offended. Any suggestions?