People how can I let my girlfriend know, so she doesnt hate me?

am a huge oklahoma sooners fan living in Norman,OK and my girlfriend is a cheerleader for university of Illinois illini, but last week I had with 2 Oklahoma sooners sorority girls which was a during a party and did not use a condom, so what can I now do? how can i let my girlfriend know? should I tell her? I made a huge mistake while drunk.Will those girls get pregnant?