I think maybe she had never really reconciled herself to him having other children (whether because he's a crap father, because she's a ginormous hypocrite, or because she's one of those people who think men - and only men - should walk away from children when the relationship with the mother ends) as well as she thought she had. She's likely loading all of her anxieties about the lifetime connection he (technically) has to his exes onto this one situation, especially when it's compounded by either him lying about it or the possibility that there may be more children out there. It's not rational, but clearly, neither is she.