Oh my, this happens to me too and it's very disconcerting. It mainly happens when I have dreams about college. I graduated eleven years ago and ever since then I've felt that I didn't take full advantage of my years at college because I haven't gotten a job that utilized my degree nor did I participate in any extracurricular activies or make any friends. I also allowed my bus schedule to dictate the times I would take classes, as I lived at home.
Even though I had great grades and did very well academically, I would occasionally skip a class, mainly in my freshman year (most people skipped some classes). If I went to college now, I wouldn't skip any classes. In my defense, it wasn't that many, only one or two a month or so, nowhere near those kids who would skip the whole semester and only show up for finals. But in my dreams about college, I will dream that I've forgotten that I was enrolled in a course, have forgotten to look at my class schedule and that I've skipped weeks and weeks of classes. In the dreams I'll show up and not know what was taught or what I'm doing.
I have these dreams so often and college was so long ago that these dreams are replacing my recollections of college, so much so that my perception now is that I was a total failure and screw-up in college. Of course rationally I don't think that I really skipped all those classes. But the emotion of failure and helplessness that I feel in the dreams is now the dominant thing I feel when I think about my years at college.