My girlfriend lied to me about a paternity test and I'm really upset about it.
Oh wow. What did she say?
She told me it was my baby, but I found out later that it wasn't.
It is awful to be lied to on such a massive level. I can understand why you're so upset. What do you think you're going to do.
I might break up with her.
I think that's a completely reasonable reaction. Lying about such a big thing is a huge violation of trust.