Now where was I, what was I doing?

No, I do not mean where was I when Kennedy was assassinated (I was in bed and asleep). I do not mean where was I when the Beatles landed in Adelaide. (I know that too. I was sitting in a Latin lesson and the teacher let the alarm clock ring.) I also know where I was when they landed on the moon. (In teacher training college watching the grainy images on the tiny black and white set they had set up in canteen.)
I can remember all those things. Well, for the first one I can remember my brother coming in to my bedroom looking rather pale and saying, "Cat someone shot the President of America." He had been listening to the news on his "crystal set".
If I can remember all those things why is is that I cannot remember other things? I can remember lots of things. My family tends to rely on my memory. I can, although outsiders rarely believe me, remember things from before I was even two. Of course I was talking by then and recognising individual words not much later. (No, I was not so damn precocious that I was reading books. I had to wait a couple of years - much to my frustration.) We need language to remember things.
I can remember vast numbers of words and ideas and wheres and hows and whos and whats and whys and whichs and whens and whatevers - and, if those words do not really exist then you know what I mean anyway. Even those unfortunate enough to have less than "average" intelligence often have remarkable memories. Knowledge is memory.
No, what I want to know is why I sometimes stand still in the middle of the kitchen and think, "Now where was I? What was I doing?"