Hi Liverpool, I think maybe you should have a chat with both of them, you may not think you have done anything wrong, but you may have done or said something without knowing, I don't get on well with my MIL because she says things about my child which annoy me, ans thinks she is welcome to visit every weekend, and other things, as a result I now hate spending time with her! I am not brave enough to tell my MIL what she is doing wrong, but would if she asked. - I would LOVE for her to ask me, because I want this all sorted out.
Your son's GF may not be trying to turn him against you, but he loves her, and with a child involved he is quite rightly siding with her, they are a family now.