@fifmack,
I have to wonder, and I don't understand why nobody brought this up yet...
Why would the ex WANT to work under his former girlfriend and the mother of his child?
Nobody even mentioned him, nor wondered about his motives. He has a vested interest in all of this as well, though. It might be that he needs the money of a steady gig, but I still find this situation a tad... off putting.
Also, both the OP's girlfriend and her ex will in all likelihood be working in the nighttime. Not sure what job the OP has, but if it is a daytime job(likely, given that there is a kid involved, who presumably lives with the mother, and presumably needs an adult to help him/her get ready for school and stuff), she's going to be seeing more of the ex than of the OP.
Personally, I think the OP is entirely right to be suspicious about her being annoyed about his subject matter.