"Help! My Girlfriend Is Acting Like A Runs Girl"

Good day, Romancelanders. I met this girl last November and things were going on well between us. But lately I've been thrown into a state of confusion as to what I got myself into. She receives calls from strange numbers and a few times, I've noticed a certain patten in her conversation, like "Hello - Yes, I'm fine - Okay, let me see if I'll have chance". and this goes for every other strange call she receives. Also recently, I found out she's been sleeping out late. Few days back, I slept out just to see when she will come back, and to my greatest surprise, up till about 2.00am, she didn't return. We quarelled for about 2days and she was crying and begging me to forgive her, that she went to see her friend, a female, though I know its all lies. To make matters worse, she has a particular friend that is a runs girl! Sometimes when they're discussing, they will talk all manner of nonsense, she will be mimicking how her friend moans during sex, and they will laugh about it. I felt embarassed and walked out of both of them.

I really don't know what to do right now. She cried and apologized for sleeping out, I could tell that she was truly sorry. I love her, but I hate what she does, and anytime I think of breaking up, we still find a way to get back together. What should I do?