Guilty Over My Actions With a Colleague

I started law school about six months ago and I've been dating casually. Nothing serious or exclusive yet. A few weeks back, I met this really cool colleague at work, let's call her Sarah. She's in her early twenties, very smart and funny. We clicked right away over lunch one day and ended up talking for hours. We started hanging out more after that, just drinks with friends or going to events together. Nothing romantic though, we're both clear it’s just being pals. But there were definitely times when I found myself thinking about her in ways that aren't exactly platonic. Last night, we had a late meeting and ended up at my place to work on some documents. It was getting pretty late and I asked Sarah if she wanted to stay over, we both agreed it’d just be sleeping in the spare room since neither of us felt like driving so late. We made coffee, stayed up chatting, then went to bed. But after she fell asleep, I couldn't relax at all. Every time I closed my eyes, thoughts about Sarah kept me awake. She's attractive and smart, why not act on it? So when the moon was out and there wasn’t a sound except her breathing next door, I tiptoed to her room. The sight of her sleeping in that old t-shirt made something snap inside of me. Before I could talk myself out of it, I climbed into bed with her. She felt so warm and inviting, and I couldn't stop touching her, running my hand along her back, kissing her neck softly. Eventually she woke up, half asleep but responding to my touches. We ended up making love that night. It was quiet, tender, nothing rushed or wild. But it's gnawing at me now. We're not together officially and I didn't say a word about anything changing between us the next day after it all happened. Just acted like everything was normal until we got separate rooms again. I don’t know if what I did is terrible or just one of those weird things people do in relationships and friendships that isn’t as big of a deal as my conscience makes it out to be. But now, every time Sarah smiles at me, all I can think about is how guilty I feel and whether she’s thinking the same thing. So here I am, feeling like an asshole but not sure if I am one in this situation. Am I the asshole for keeping this a secret? What should I do from here on out?