The exam is coming and sometimes i feel unhappy with no reasons!i am worried that this would lower my marks.what should i do?thanks!

Worrying that the: exam would lower your marks IS a good reason to worry. This sounds like test-taking anxiety, which is VERY, very common in all walks of life, not just school. Over-rehearsal of the information being tested seems to be one of the best ways to lower this kind of tension, but if it does not subside post-test, you might have a bigger version of anxiety that needs TX. If so, see a psychologist.

Stress!! : Wow, exams can really be stressful! Take care of yourself, get plenty of sleep, try meditation and yoga to relax your mind. Just do your best. If your anxiety is interfering with your functioning in relationships and life, definitely see a counselor for help!