[Jess~]

yeah, i'm almost 98% sure you're not actually fat.
bloating and weight gain during your period is completely normal and you shouldn't be criticized for it because you have no control over what your hormones do to your body.

i'm going to take a wild guess and say that the weight gain isn't too severe, judging by the other things your mom is saying about you. if she was being gentle and genuinely concerned, i might think you should be worried. however, the fact that she says you "ruined" yourself and is calling you big and whatever, that tells me that she's extremely exaggerating the state of YOUR body.

i bolded "YOUR" because it's important to remember that your body belongs to you and you alone. nobody else can tell you what to do with it or whether you should change it, that is entirely up to you.

i tend to eat unhealthily while i'm on my period or a few days before my period as well. again, that's just our hormones giving us cravings and telling us things that we can't control. of course, you can control what you put into your body, and it's good to be aware of that and try to stay healthy. however, you're 15. you're still a kid. you can eat unhealthy things and not have it immediately go to your stomach.
my guess is that this "weight gain" and bloating is more on your period and less on the food you ate.

if you're concerned, and want to work to lose this extra weight (again, my guess is that it's going to go away shortly after your period ends), i'd love to help you design a workout schedule and give you advice on your eating habits. but that will take a lot of time and effort, and honestly, i think you're most likely perfectly fine.

there's not much you can do between now and tomorrow afternoon to "not travel fat". you could always try googling ways to get rid of bloating, because that would help the most.
my advice is, just stop worrying about it, at least while you're traveling. have fun, eat what you want, just enjoy your day. forget whatever your mom says. if you have to, tell her to take a look at herself before she tries judging you. often times people who are ugly on the inside will try to make other people feel ugly on the outside.