Why do girls catch feelings after a one night stand?

Anonymous
I'm pretty cold hearted at least because of my past. I can move on from guys really fast (before I had emotional attachment issues). Anyways I hooked up with a guy and had a pregnancy scare for 8 days. I had no feelings for this guy (hardly knew him), my friends even asked me if I wanted to date him and I said no. After all this I don't know why but I can't stop thinking about him. Does this mean I caught feelings?
TMI but it's the time of the month so that maybe messing with my emotions as well.
Updates
+1 y
I lost my virginity to his guy because I lost faith in love and wanted to see what the hype was about. Also I wanted to lose is before I turned 21
Why do girls catch feelings after a one night stand?
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