Is it normal that it went this far with an immigrant student??

I've been helping an immigrant student from Ukraine with her English. At first it started with her asking me a few quick questions because I just happened to be sitting closest, but soon I was regularly looking for mistakes and proofreading her essays. I was never assigned to tutor her or anything like that, but I began helping her often. This started around three months ago. A few days ago, I was at her apartment, and I was about to leave but she asked me to stay for a bit because she had something for me. She went off for a moment and came back in a nightie. I was surprised, but her smile, the way she moved closer, and the dancing she did with the sharp movements of her hips and hands clasped above her her head made me see what was about to happen. She got close and put her hands on my sides and I asked her if this is what she really wanted, and she said what's important is that it's what I wanted. Well, I am a man, pointless to deny anything there. I then told her she didn't have to do this and she said she wouldn't mind it, then laughed and told me she had to compensate me somehow. At that point my self-control was completely gone and we had sex. When I was leaving in the morning, she implied that as long as I kept helping her this kind of thing would continue happening. Since then we haven't talked about it but I think she's smiling more often and leaning in a little closer when I'm looking at something.

Was that irresponsible of me? Neither of us is in a relationship so it wasn't cheating. I was also never actually designated as any kind of tutor, so it's not like I'm abusing a position of authority. But still, I feel like by having sex with her I crossed some sort of line. I was honestly surprised when it came up—I didn't really think I saw anything between us—and I can't help but feel like I've done something wrong. That she's a very good looking woman—not surprising, as she is Ukrainian—doesn't help my judgment. Even though I'm worried, I just know the next time we're at her apartment and she gives me that smile, I'm not even going to be able to think twice. I'm really conflicted. The emotional side of me says I'm abusing her somehow, but the rational side says that I did nothing wrong, I went with what we both wanted, the way we were getting closer with time and going more more places together was leading up to this and might as well be considered a date, this is the closest thing I've had to a stable relationship in a few years, I'm not a real teacher or tutor so there's nothing unprofessional about getting close, there's no good reason to be worried about it...

Help me out here. Hasn't this been the plot of more than a few sitcoms? Life does imitate art sometimes I guess. Sorry, I know I rambled a lot in this. Did I do something wrong that night or am I just worrying too much about things that don't really apply?