In My 50s Again With A Twinge Of Loneliness

I've been teaching at a university for over two decades now. After my divorce five years ago, I started dating casually again. It's nothing serious, but that's okay because it doesn't feel like anything more than that is possible anymore. Recently, one of the graduate students in my department asked me out. We have coffee and catch up once a week, it’s not romantic or anything, just friendly conversation. She always seems so eager to talk about her dissertation progress and asks for advice from me now and then. The other day, we found ourselves alone at an off-campus event. There was a moment when she looked at me with such intensity in her eyes that I felt my resolve start to waver. We ended up going back to her place together and... well, it happened. Afterwards, there was this weird tension hanging between us. She said nothing about what had transpired and has barely spoken to me since then. Am I the asshole for taking advantage of a student? Is there some unspoken rule that stops a teacher from ever getting close like that with someone so much younger? What should I do now?