You could approach this problem, which is universal BTW, in infinite ways.
A classical, or Freudian if you will, would be that subconsciously you actually don't want to write, hence your brain is simply listening to your desires.
In a more modern view of the mind and brain, you could think of a metaphorical bucket of resources, which is always being drained by your activity and simultaneously being refilled. When you keep the level of drainage and refilling at equilibrium you are basically on your average game. But certain actions tend to drain your resources a bit faster, and afterwards, you need to compensate so that the bucket can be refilled.
Your statement that there is no correlation between what you've been doing before and how dumb you sometimes get to be afterwards is a bit of an understatement. There is a correlation, you just don't see it.
But I would take a wild guess that you are looking for one when you observe that you are not at your peak, which is not exactly an ideal time to expect omniscient revelations.
Also, so I don't get your hopes up that everything will be OK if you just wait for the bucket to be refilled, let me just point that the pool of resources that is refiling your smart-bucket is actually finite. So things will get worse as you travel further from your point of birth.
Or you could have been dropped on your head as a child...
Hmmm, so much to choose from! Of course there much be reasons for my dumb days, but on a surface level, they are not easy to spot. That being said, I'm not too worried really, I just take it as it comes, and try to work more on the days when I have the smart brain ON. As in everything, it all just takes more practise, and my brain sure needs it.
I would like to just blame something that happened on my childhood, isn't that the way to go these days?
Of course - you and the group that you identify with are oppressed and therefore you are unable to express yourself.
And me being a white privileged male pig, naturally didn't even considered that you are suffering because of reasons...
No one really understands how hard it is to be a skinny white girl in a western society, the struggle is real ya’ll!!
That's why you have me to mansplain everything ;)
What would I do without you!?