What's wrong with me?

You mention exams, so it sounds like you could be going through a stressful time in life, you might be suffering from some anxiety and mild depression, it can certainly cause our thought processes to go haywire. Trust me I know:) If you are going through exams and such, see how you feel once they are over, if you aren't, or it doesn't get better, I would contact either a counselor at school or another mental health agency and talk to them.