I'm sorry that happened to you. Most of the people at conventions are really nice and fun (as they are all running on anime and sugar, lol) but if you run into a mean one, it might just be better to not hang around them. Usually I would say you might have caught her at a bad time but if it was like that all weekend, It just might have been her :<
And yes, Like DJ Bullock said, Try to be polite about it or you'll end up paying for their rudeness.
On that note:
there was one girl when i asked to take a picture she yelled... kinda sucked. anyways, just dont let it get to ya~
Most cosplayers are really nice and like to have their picture taken and while I'm fairly certain you didn't do this, just as a side not for everyone who might have had this happen.
Try to be nice to the cosplayers. If you run up to them and flail about while yelling at them for a picture while they were trying to eat, or having an important conversation, they probably wont be as nice to you as if you had come up and politely asked. And as cosplayers are human, some times they just have bad days.
everyone just needs to have a balance of politeness, tolerance, and courtesy.