Hi Melisana,
I just finished my Master's degree and am starting a PhD program in the fall. I also had trouble concentrating. Sometimes the simplest things just would not make any sense. I If something does not make sense, it is important not to stress about it. Stressing about it makes it worse. Try not to overload yourself - that is hard to do in grad school. Also, take frequent breaks. It is good to pace yourself - do a sitting and reading/writing task for a little while, then switch to a moving around task. The student disability center might also be a good source for you.
Good luck!
Sue