If you didn't know her when you moved in, how do you know that she "became a different person"?
Did she stiff y'all on the rent?
And all I can say about Sorority Life is...when you were living with your roommates, did you stay there rent free? Did you have drama 24/7? Did you have a camera crew filming your every move? Did you not have radios or TVs? I didn't think so. Life is not like an MTV show - ANY MTV show.
And like ktsnake said, duh - college guys want to get laid. It's the same everywhere, not just fraternity parties.