When anyone experiences any sort of emotion which contorts their face, the image of their face will remain in my head and I'll analyze their physiognomy. I often do it to my own reactions. If it is anything embarrassing it can keep me up at night.
I do this too! Exactly the same.
I work in retail too so I have to talk to a lot of strangers and for example if I look at them and they have a certain look on their face I fear it's because of me and then that image is stuck in my mind for the rest of the day and night while I work out why their face looked like that.
I'm trying to get around it by just telling myself it's impossible to know why they looked confused/happy/angry/weirded out and so there's no point thinking about.
Of course that doesn't stop the problem it just prevents me from thinking about it as much.