aggressive how? Pits are a dog a beginner/first time dog owner should not have. I love them, they're great dogs, but they are strong and bull headed. They need an alpha owner to make the relationship work as it should. If she being protective over you, (people walking near you, touching you, etc.) you need to become alpha quickly- she's claiming you as hers. She should not go in or out of your house before you (you walk out first.) She shouldn't be able to eat until you tell her she can. She should not be allowed on the furniture without permission. I'd also get her a gentle leader; it's a head halter, it works similarly to a horse halter. It helps with lunging/ jumping/ pulling- she could only pull you with the weight of her head. Pits are notoriously good guard dogs, but YOU need to set the boundaries of when being protective is what you want, and show her you're the leader of your pack.
If she's being aggressive with you/ in general- without any warning/for no reason, I'd take her back. They shouldn't adopted out an aggressive dog (if she's from a rescue.)