What are you eating as a vegetarian? Beans and rice? Or brownies? Does the school have true vegetarian options (as in, they make the food vegetarian by design), or are you picking around meat? Can you find out what's in the food you're eating, and the calorie content?
Where are you in your cycle? Surely you have noticed your weight fluctuates all over the place every month. Water weight gain can be combated by using less salt and drinking more water. Yes, you'll pee more. That's the bane of many a dieter's existence.
Alcoholic beverages, if you indulge, can be big-time calorie traps. And even diet soda is a problem, as it often tricks your body and makes you crave more food. Restaurant and other processed foods are often variations on a salt, sugar, carb, and fat theme. If your school is serving foods straight from ARAMARK or another big chain, it might be undergoing the same form of preparation.
What sort of exercise are you doing? Cardio? Weights? Something else? Talk to someone in authority at your school's gym and find out if they have recommendations. There might even be a class you can take for credit (back in the day, my school awarded half a credit for a semester of swimming, for example).
Keep in mind, too, that even setting your cycle aside for the moment, weight fluctuates a few pounds here and there no matter what you do. Some of that's water, some of that's legit weight issues. An easier way to figure out what's what is to track over time. This means not only your weight but your measurements. There's the standard neck, hips, and waist for BMI, but women should add in bust as it definitely changes depending on your weight. And BMI, by the way, is out of favor as a true standard. Go by how your clothes fit. You may be gaining muscle, which is denser yet smaller than fat.
Also, track your food! You may see where you think you're eating better, yet you're not.
And when you're my age (I'm 57), I suspect you'll have the same thoughts that I do, which is that I wish I was as "fat" as I was when I first decided I was fat.
Standard caveat applies: I am not a doctor and suggest you see yours.