Why am i getting stupider?

I am 16 years old, finished at the top of my cl with A-s, went to the best scool in my country and now im getting stupider. I work out, do crosswords and sudokus and such regularly. Lately i have been having difficultys understanding even the simplest of stuffs, sometimes i cant even understand what the other person is saying in a regular conversation,i re-read material multiple times before understanding what it says and sometimes i immediately forget it. I used to grasp things perfectly and use them immediately, now i cant even solve tasks i have solved before! My head feels constantly thick and most of the time it feels like my head is in "clouds". I dont do drugs or drink.I even had problems writing this letter.Could anyone say what am i doing wrong or should i see a doctor or something like that.