Half of the times I cannot concentrate would be the times my sinuses bugs me for every half a minute.
And the other half would be the times of receiving too many irrelevant information (explicit or otherwise), I'd be confused due to constant uncertainty.
The bottom line is usually what the latter basically is; receiving something too many irrelevant stuff, and become confused by being too uncertain.
And thus, this confusion would make me ask stupid questions and do foolish actions.
That's how doubt is born for a day, only to disappear later. To only appear again, just to disappear again.
Therefore, yay! I have a weird inconsistent cycle of competence and incompetence because of it.
So yes, I have those days.