Alot of growing up has probably occurred within the last 2 years.
It is good that he is being so upfront about this girl. If they are going to have your DD she might want to know about you but she also might have heard horror stories from others about meeting 'the exwife' and not want to go there.
Sure, you are hurt and will ask the why questions but if you know the breakup was for the best you will have peace of mind.
Hope it all works out for you both