Does this girl have a right to be mad at me?

This story may get confusing, so I apologize. My boyfriend Jake lives in a frat house. Nicole, who isn't dating any of the brothers, is always at the house. She cooks, she cleans, she buys them alcohol, she hangs around, she has with them all, she is basically the house slave. Anyway, Nicole's best friend Anna started hooking up with Jake's frat brother Billy. I've met Anna and Nicole more than once and have tried being nice and they are just that "fake nice" back to me. So, one day I bring my friend Lisa and she hooks up with Billy. Now Anna is mad at me for not telling my friend that Billy was already hooking up with someone and to back off. Should I have done that? It's not like I am friends with Anna or Nicole and they weren't dating, just casually hooking up. Now Anna is being a huge ***** to me.