YTA Your lying about something that many people face and can be devastating rather then sit down and let your mother know that you dont want to have children right now.

The one thing I can say is you went with the nuclear option you definatly won this argument and she wont be bothering you about it again. She most likely feels awful about asking you so much, and you can always tell her in the future that you dont want to talk about it and she will probably listen thinking it's just too painful.

Here the thing it you were pretending to have cancer you would be the asshole then too. Many people suffer from infertility and its dehumanizing, humiliating and painful. Your using their pain as a shield to shut your mother up. And that makes you an asshole.