Why's my mum so backchatty?!?

honestly anything i tell her in cnfidentce, she goes and off a bitches to my dad and brother. I only even found out yesterday, I indirectly called one of my friends unfit wihch I didnt even realise at the time(this was a good two months ago), but I just wanted someone to encourage her to do some sport (none of my friends do ANYTHING!), I think I commented something like 'you dont want to be one of those unfit girls w no excuses' she took it wrong anyways - but i was in an arguement w my brother and he brings that up, telling me mum told him.

It just bugs me, I dont normal upset anyone, and I dont really b'tch at all, yet my mum does - to my grandma and her friends about MY problems, i tell her and trust not to pass on, as I need advice,

What can I do to stop her!!