Can anyone tell me what's wrong with me??

I am having my af on Friday. Feeling all the symptoms that of pregnancy.. But even my blood test Came up with a big negative result, which is obvious that I am not pregnant.. So I am worried whats wrong with me?? I feel really dizzy, can't stand for more than 5 mints, feel tiered and low and cramps in tummy as well. Does anyone has any idea about my problem?