This is the first I've ever heard of any flashbacks that are not related to severe trauma. It may be that "flashback" is not the proper term but I can see how maybe some memories might come in a similar fashion to the way flashbacks do.
You mentioned that this often occurs right after sleeping and it occurs to me that it may be the activity of your subconscious --unremembered dreams and what not acting on your conscious experience in some way.
I have a lot of strange experiences with my mind. De ja vu, certain smells activating a flood of emotions or experiences such as what you described.