Salaam,
Is something troubling you? This used to happen to me, especially when I was agnostic. When I was studying, I started to worry about other issues and could not concentrate. It got really bad because I could not sleep at night, was getting late to my lessons, missing homework deadlines, very close to falling asleep in class, eating all sorts of junk food and extreme guilt/sadness/anger. I thought I was going to have a mental breakdown and beat someone up.
If you have underlying issues that are affecting your studies, then do try to resolve them.
It could be that your studies have gone much more difficult or your bored. It can get boring to study the same subject...especially after 3 or 4 years. :skeleton: