Even if she hadn't lied, contraceptive potions and charms and what not have been known, on occasion, not to work properly. Which is why -- look, I agree that what she did wasn't ethical but you can't deny it takes two.

What exactly does she want from you? It's not as if you have some stable or simple life you can whisk her off to, even if the child's yours and even if you want to. (And yes, I got that she says she doesn't need or want or expect anything from you, but that's clearly a lie, or at least a game, not anything she means. You must have some idea what she wants.)