Kia's 100% right. Plus it's not like this is something you can hide very long. And the longer you go without telling her the harder it's going to be and probably the more she's going to go off, only then you'll get a bigger lecture about why you didn't tell her sooner. Yeah, it's pretty scary, but the best thing you can do is get it over with as soon as you can or else it's only going to be more stress on you.
My dad always says you have to give people a chance to rise above your expectations, and that goes for your aunt. She might surprise you and not go off like you expect. But even if she does go off, I agree that the ebst thing you can do is not to react and give her a chance to soak in what you told her and what you're going through.
I was sure my parents were going to kill me, and sure they got really upset at first. But then they also settled down and they've been really helpful and supportive. And believe me, this is a time in your life when that's what you really need. But they can't help you til you let them know what's going on.
Good luck!