Can I tell my girlfriend that she has offensive body odor after sex?
Are you joking? Why would you tell her that?
Because her odor makes me nauseous and I can't eat
You shouldn't wait until after sex. That is really a hurtful time to ask. Maybe bring it up calmly in the morning.
I don't know how to bring up to her, it's been a bad thing that I have to solve now.
It's certainly a sensitive subject and I can't imagine that the conversation will be easy. Try to be as sensitive, supportive, and as understanding as possible. Be mindful of her feelings and don't let her think that you think lesser of her.