What should I do about this?!

Good god relax.. Your mom should too. I've known girls who didn't have it until they were sixteen. If she does insist on it then don't worry a gynecologist is still just a doctor. But she shouldn't make you go if you really don't want to unless it's clear there really is a serious problem. Not having a period yet is not inherently indicative of a serious problem, everyone has it at different times. I think all she's doing is stressing you out and making you think there's something wrong with you, and probably inadvertently making you feel pressured to have your period which is completely out of your control.. I would have her call a doctor so they can tell her this too, because I can guarantee that no doctor will say there must be something wrong with a thirteen year old who hasn't had a period yet.

and what exactly is the gynecologist going to tell you? your mom needs to chill out. you dont need to go unless you are having some kind of symptom. does she go to the emergency room and say "doctor, i'm feeling terrific today, whats wrong with me?"

I don't think there is a reason that you would need to go to the dr about not starting your period. I started at 11, my sister at 9 and my cousin at 16. Every woman is different. If you don't get it within a few years then I would go to the doctor.

dont worry about it, everyone gets there periods at different times in their life, from the age of 9-16 on average.