Dreaming About Former Stress Experiences

It's a cliche that in the industrialized world, adults tend to dream about missing exams in high school or college. You know the ones - you have a test or a class you have to be in but you don't know where it is, the hallways don't make sense, your school or the buildings aren't the way you remember them, and it's a disaster. You wake up and you're maybe even mildly amused; thank goodness you'll never have to deal with that again. Why do we seem only to have these dreams when we're no longer in that situation?

After a 13-year hiatus from education I began medical school at age 35, leaving a career in biomedical research consulting. At one point prior to starting med school I joked to a colleague that I could no longer laugh off those missed-exam dreams because there are many, many more quite real exams in my future. But what I am amused by is that now, I no longer have missed-exam dreams, and instead I've started having anxiety dreams about situations associated with my pre-medical school career. Usually, in these new dreams, I'm arriving late at an airport only to find out that my flight has just left. That this change occurred less than a year after I left that career certainly suggests that there's some mechanism which pegs anxiety to stress experiences that we no longer have, or that are associated with some earlier phase of our identities.

What could be going on here? There are lots of people that change careers and/or go back to school, so has anyone else had this experience?