How can I stop a three year old hating me??

Wrkin in cezch republic as au pair. 1st week was fine n its gr8 with 7yr old bt 3yrold girls bhaviour 2wards me has rapidly deteriorated, now she wnt even let me undo her seatbelt, shouts in my face that she wants her mum. so wenever mum pops out (she usually stays home 2 learn eng from me) grandfather comes over as otherwise the girl screams the whole time. im quite young (17) but have lots of experience with kids (mainly special needs) and neva felt so unsure of my abilities. tried tlkin 2 mum bt she brushes it off, says girl needs time. bt she was fine 1st week n 3yr old nxt door loves me. i have 2 say shes a bit spoilt but shes only 3, it shouldnt b this impossible 2 win her over. im certain she feels safe with me, just dislikes me. she gives every1 the attitude i get bt 2 others shes often sweet as well which i neva get. i hate feeling like im taking adavantage of a lovely women as i cnt even take kids 4 a few hours. i get the feeling mum struggles with her too. Advice plz....