Husband is about to catch it...

I got married August 17, 2018 and my husband had kids from a previous relationship. Me and the kids mother was never on speaking terms and I always thought of her as an evil miserable bitch but today we finally had a conversation and she told me the reason she was upset was that they didn’t break up until July 2018 and she didn’t find out we were married until December 2018. He had cheated on her in 2014 after she had their son. He never told me that so now I’m lost and trying to see if I should believe her.