Sorry to hear this OP.

It really sounds like you mum didn't know how to handle the situation and went straight into nuclear mode. She could have taken the opportunity to sit down with you and discuss it. Could have been a bonding experience but instead it's created a divide.

If I were you in this situation, if my grandparents went straight into siding with my parent without asking my opinion / side of the story, I would probably just tell the grandparents that with all due respect, it's none of their business and that you don't want to talk about it with them.

Good luck and I hope things settle down for you. I think give it a few weeks and you mum will probably forget all about it.