its probably a number of things. She is most likely a bit jealous about the fact that her ex is now having a baby & going through parenthood with someone else (somethng that he only shared with her previously). She also may be interested in getting to know you more due to the fact that the kids will be related. Either way, do what is comfortable for you & do not fall into the trap of letting everyone get too lovely dovey becos it usually ends in tears - its better to have a little distance between the ex & your family.
Good luck w your baby & make sure you are true to yourself & your feelings if you start to feel uncomfortable about the situation.