I am wondering if the age is not part of this process as I have seen this in my 10 year old son this year. Forgetfulness was never part of my son’s repertoire, but around October (this year, 4th grade) suddenly he began forgetting assignments and small duties left and right! He also had become very withdrawn- barely talking on the way to and from school, staring out the window, barely listening to me.
With our experience, you hit the nail on the head when you said “increase in work and change in atmosphere related.” As my son was becoming less happy with his classroom/school environment, he was developing a case of the “I don’t knows” and “I forgots.” When we drilled down into what was bothering him and after meeting with the teacher and principal to get things on track, the forgetfulness and spaciness subsided. I do not think that you are misreading your son’s symptoms at all and it is definitely not a stupid question.
Good luck to you and as always, hang tight. This is a great site for feedback!