Hi , I Am A Teenager And I Am Psychologically

hi , i am a teenager and I am psychologically having some simple issueshttp://www.healthcaremagic.com/web/images/ui/consultNew.png as when sometime some person ask me question (about science or computer i am science student) I know the answer but i could not explain to others it as I know myself and sometime I suddenly forget the things I do know already and made a mistake please tell me my problem or disease or problem.