I get like that too. Normally though about small things that I make a big deal in my head. Like the way I did something that my preceptor would do a different way. Stupid stuff. I find it is worse when I haven't taken my anxiety Meds correctly or if I haven't slept well. I guess it's something we just have to deal with. I can usually tell myself that it wasn't a big deal and the pt was ok and I will be fine enough to go to bed. Sometimes I need medication to calm my mind. I think it's normal to feel this way sometimes though.
I think it happens to everyone at some point or another. It happened to me a lot when I was brand new, and again every time I've changed jobs. It happened to me when I was involved in a sentinel event. And it happens whenever I'm under a lot of stress, either at work or in my personal life. Journaling helps.