Looks you need to have a chat with someone. That being said there is no certainty the kids share a father. Maybe a paternity test is in order if to confirm she is delusional or some one has been jumping the fence. Either way, good luck OP.

Maybe she fancies your husband and is just trying to split you two up? Some people can be that psycho. Don't believe anything or assume until you have cold hard proof that the baby is your husband's and if it is then you have perfect grounds and proof for divorce if you wanted to go down that road. Either way this must be so hard on you and I'm sorry for the situation you are in. Best of luck for you and your child and hope everything turns out okay.