As I noticed, after reading 2-8 thousand words a day, after a few days of reading like this, it becomes very difficult for me to read, and I started reading actively lately, before that I only watched TV shows and movies, and now I have a slight headache, the text is floating, I even confuse words. In fact, that’s why I used to read very little, and now I’m so tired of TV shows and movies that I’m willing to force myself to read, even if I have a headache, to become emotionally attached to books instead of just consuming like zombie like I used to.
I’ve been there, with a mild astygmatism that went undetected for decades. The only signs were headaches and trouble focusing after looking at the same thing (books, computer screens) for a long time.
Schedule an appointment with an eye doctor to have your vision checked, and explain this issue. Our eyes and brains are very good at compensating for minor problems, and it can even slip by an eye exam. But looking at something close and fine, like a book, for several hours can tire out and overwhelm the eye muscles, until they ache and stop working well, unless you rest.
If you live in a dysfunctional nation without affordable access to an eye doctor, a pair or reading glasses may help by making focusing easier.
Good idea with reading glasses - of they make a noticeable difference then there’s something amiss.