What Causes Sudden Flashbacks Of Bad Memories?

I ve been experiencing something I don t understand, it starts with a flashback of a bad time in my life, I try to remember in detail what it is but it goes away so fast. My whole body gets a feeling of rush. My head feels strange and I have to sit because I feel weak in my legs. It subsides as soon as I sit. My heart beat isn t affected. Then 1 to 2 hrs later I get another one but not as bad. I ve had up to 6 in one day, each getting less of the symptoms. It s usually when I m stressed and a lot going on or I think of something and boom there it comes. Sometimes months will go by without one. Last one was a yesterday but mild, and only one. I get totally exhausted after, but depending how bad one is.