My mum is talking to my ex girlfriend, seriously wtf?

Keep anon please.
Basically my ex left me a good 4-5 months ago now.
We have both moved on, she has a new boyfriend and i have a new girlfriend.
The thing is, my ex and my mum used to text eachother sometimes about stuff, like if she was upset or something she would text my mum.

I dont want to hear about my ex anymore, ive moved on and i dont wish to be friends with her or anything.
Well, i come home tonight, my mum goes, 'ive been talking to Sarah'.
I ask what about and she says my ex has got a new car and stuff. Then they were discussing her boyfriend and other things. I ask to see the texts and she says 'No get lost, she's my friend its nothing to do with you anymore'.

She's hidden her phone.
Now i am either being really unreasonable, or i have a right to be very pissed off at this. because

1. Why is my ex suddenly texting my mum about something as trivial as a car, who cares? Who would tell their ex's mother about this?

2. She didnt even get on with my mum that well anyway.

3. My ex must have known full well that i would find out they have been talking.


I'm annoyed because i cope best when things are totally out of my life that i no longer want to be involved with.

Any opinions?

2. She didnt even get on with my mum that well anyway.

The funny thing is its pretty obvious my ex is using my mother as a 'middle man' to pass messages on because she cant pass messages from herself to me without it being ridiculous. 'Hey, btw my boyfriend is 6ft 4 etc etc'. So she is telling my mum these things as she knows my mum will find out.
It is a form of passive aggression i think...
What to do about it, hmmmmm

Be the bigger person and ignore it?

Seems like she's baiting you, don't fall for it and get pissed off, just say "oh cool" or "that's nice" when your mum talks about it even if you don't mean it - as your mum could be texting your responses back to your ex for all you know.

All it shows is my ex still isnt over me if shes texting my mum.