She was overly capricious that nothing would happen. You could tell by her body language that she didn't have a worry that something negative could ever happen to her, because it never had before. She was so happy and unknowing in the "first" (last) scene in the film.
After the rape (if she survived), her entire belief system would change. She'd be completely traumatized, and I doubt she'd ever even leave the house again. The incident was "Irreversible".
Seriously, I have to say that was one of the best performances I've ever seen. The pain and horror she went through and the way her life completely changed forever, in the span of 8 minutes, was so convincing. Excellent film.
She was kind of inspired by that dream she had and the anger she experienced with Marcus. Plus a prostitute told her it is safer than taking a taxi and she didn't really see any reason to disagree. And maybe she kind of felt curious if she could walk in and out of the tunnel that nothing really terrible would happen. I don't think she had any idea that she would come across a psychotic pimp who would then manage to sexually assault and almost nearly kill her.
She was probably too pissed off at her BF after the fight to think clearly. Plus, that dumb broad told her it was better to go that way, so she assumed it was safe.