What is the problem?

Yep I've had yet another argument but this time I just think what's the point? I mean I think that every time and yet she gets back into my head and give it another go and things work for a few days, weeks and even months.
Tonight I got a text from my first ex wife asking if my son was calling her tonight. To me that's an innocent text with nothing to it but my current gf objects to this. She believes she texts me when she's bored or has no company and says that I should tell her to go away.
To me the text is harmless and yet as per every text it causes a big argument. Tonight I'm back on the sofa because I've had enough of it. I don't see her problem. She says she texts me constantly but there's been 20 texts since March mostly about my son.
I pointed out that when I was with my second wife she'd text 20 times a day. It put a strain on things so my wife at the time would reply instead and then she'd text her instead.
Yeah my first ex wife is a pain in the arse but it's not how it used to be. It certainly shouldn't be causing the all out war it has tonight. I just think what is the point and finally saying we're done. What do you think?