"that girl" refers to any situation you don't want to be in.
1. A girl who is overly obsessed with her boyfriend
2. A girl who blacks out at a party by 8:30
3. A slutty girl
1. They've only been dating three months, and she's already become "that girl". They spend every moment together.
2. She showed up and blacked out in the bathroom, it would suck to be "that girl". Luckily, I was "that girl" last night.
3. She's messed around with seven guys this weekend, I never want to be "that girl".