Memory fails you after stress
Stress makes people much more likely to create false memories, say American researchers. It also appears to make them more certain that these false memories are correct. There is serious doubt on the reliability of victim testimonies in cases involving psychological trauma. People are woefully bad at recalling details of their own traumatic experiences. This could help explain why crime witnesses give conflicting evidence or pick the wrong man in a line up.
The brain is affected by stress hormones. The hippocampus, a brain structure needed to form new memories, is riddled with receptors for the stress hormone cortisol. Cortisol may cause the effect by suppressing the hippocampus. The nature of the false memories suggests people might tend to let stored associations influence their recall of a stressful event. It might be possible for memories of a crime scene to be influenced by beliefs about the sorts of people who commit crimes.
If you can compare stress to a sort of trauma, it might be the explanation to why stress makes you forget about things.
There is more here:
Memory fails you