My son's mothers behaviour is starting to worry me.?

At first I was going to say it's just hormones but when you said she has a past of doing this,Sounds like you need to talk to her and explain that you want to be there for her and the baby. Maybe the first guy she got pregnant by didn't and now she's thinking this will be the same sort of situation so she's putting up her guard. If talking to her doesn't work and she still continues to be disrespectful then you may want to get some kind of custody agreement worked out through the courts. This baby is yours just as much as it is hers and she needs to respect that. Some women think because they are the mom then they get all the say so and that's simply not the case.