I'm just speculating here, but maybe her negative attitude stems from your own attitudes or behavior regarding your healthier choices.
Do you talk about it all the time? Do you look or act sad when others order food you no longer choose to eat but still wish you could? Do you lecture others on their poor choices? Or do you simply raise an eyebrow and look disapprovingly at their choices? Do you put your foot down about where the family is allowed to go or what restaurants they can eat at based on your own diet choices?
It's very possible that what seems like a positive direction in YOUR life can feel less than positive in the lives around you, depending on how you go about it.
What difference does it make to Mom if you happily order grilled chicken and steamed veggies while the rest of the family has fried catfish and french fries? It WILL make a difference if you act like a martyr when doing so....or if you look down your nose at the other dinner choices.....or....
Maybe she feels like this is HER vacation and she doesn't want it spoiled by feeling like there must be some conflict surrounding every meal.
You don't have to compromise your own healthy choices....but you CAN promise her that you'll relax about the whole thing and enjoy the vacation (and allow everyone else to enjoy it) just as much as if you weren't "on a diet."