i think its best to tell your mum now , before it gets worse.
your an adult and so is she. it can be sorted out , one party might get a bit petty , but life isnt all roses and if she has a cry , explain it to her . and then leave it at that , this isnt about you its about your DD so your mum will have to suck it up . its your daughter . be strong , be her voice untill she has hers x

i was in a situation where my mum and i were at loggerheads over my parenting , she got really upset , but only because it wasnt going the way she wanted it to . but i kept my ways and eventually she apologised ..

hugs