Thank you very much for the replies!
The fact is that this does not feel like a normal laziness, that's why I wrote here. When I was in a normal mood I could study with no problems, no matter the location, either the library or at home. This feels more like what I've read about the "Writer's...
I have just finished my first year in Physics, and in a few months I will start the second year, and although I have a lot to study I feel I am stuck when I get to solving problems. Its not like I can't solve them, but I simply want to do something else entirely when I get to the problems...