Did she make a mistake and is she sorry? Not because she was caught but because she truly regrets her actions? If so, talk it out and try to put it in the past. If she is a chronic cheater or if you can't forgive the betrayal, I'm afraid the relationship is broken beyond repair and you should try to move forward without her. I am sorry for your pain.
Your relationship is obviously in trouble. Are there communication problems or any problems between the two of you that need to be resolved. Most time people cheat because they are acting out. (its childish but true) If that is not the case (and your very sure) then the relationship needs to end.
Girlfriend? What girlfriend? That's what you should be saying. Don't let someone play you like that, she's taking advantage of your kind nature & the fact that you love her. Now I'm not saying she's not sorry, she may not even ever do it again but the trust is now gone & with that there's nothing. Do you want to spend your time wondering where she is & thinking she's lying even when she's not? Let it go, don't let her trick you twice.
Can you have a healthy, normal relationship if are worried when she is late or doesn't call back right away? Will you enjoy trying to sneak check her phone or following her around in EVERYTHING she does, because this is what's likely to happen. Without knowing your history i think it's better to move on to someone that is considerate in how they treat you.
Unless she has a really, really, absolutely fantastic reason for why she did it, you should leave her. You can't have a relationship without trust, communication, and respect, and she's singlehandedly destroyed all three of those things.
These are the treats that come with cheats, let's see if you want some. #1-(((aids))) #2-(((gonorrhea)))#3-(((herpes)))#4-(((chlamydia)))#5-(((syphilis))) just to name a few. Listen to me my brother the treats that I just named don't love any body nor will they forgive you, stop playing russian roulette with your life because when the gun goes off that darn bullet will not return. Wake up bother and start loving yourself this is your life we are talking about here, not a video game. You may love her but her actions says that she don't love you. And besides no one wants a cracker jack box relationship with a surprise (((STD))) waiting inside of it and life is to short for (((( TRICK OR CHEAT ))) relationships because it's not HALLOWEEN EATHER!!!!