This happens to me. I can't even remember what I read on the previous page so I just close the book and try again later. Sounds like ADHD but no, I don't have that. I just can't focus anymore. I'll be reading and thinking of something bad at the same time. I find I do this more when I'm not truly interested in the book. I did this a lot in grade school. I hated social studies so for the life of me I couldn't grasp what I was reading. I'd have to go back and re-read everything.