Unqualified opinion.
A nuclear cataract is often the reason for sudden myopia. Due to the fairly sudden onset, I wonder if such a problem might be causing such a swing in the CYL. I don’t know, but do have a detailed inspection for this problem.
The strange thing is, that with a nuclear cataract one can see very well with correction…for a while. Then the brain starts to look at various paths round the obstruction. i.e. multi-lensing. At this stage there is no option but to have the lens removed.
This next comment is really thinking ‘outside the box’, but I have just met someone with this problem. It was clearly observable to their family.
I expect you have had an initial checkup, but if there is no significant discomfort, have someone that knows you well, make a close inspection of the shape of your eye. By this, I mean the type of observation an artist might make rather than an eye doctor. Swelling due to chronic sinus infection in some swimmers for instance, has left some people with a pressure that distorts the entire eye. It might be very slight, but noticeable to a close relative for instance.
It’s strange, but people do not seem to feel more than a very slight pressure in some of these cases.