I really care about her, but she just hooked up..what should I do?!?

If she's dating somebody else, you can't ask her out. You'd be showing her that you don't respect her. Keep being with her. Tell her things like, "I can see what those men see in you. You're very pretty." See how she responds. At some point, you will have to tell her, but don't ask her out when you do it. Look her in the eyes, tell her how pretty you think she is, and tell her that you're crazy about her. Pay close attention to her reaction.