What Causes A Delayed Period,nausea And Dizziness?

Ive been feeling very nautious for the past few weeks at different times of the day. I'd be really hungry one moment and super nautious the next, especially if I'm smelling food. At first I was throwing up, but now I am only nautious and do not actually throw up. Ive been getting dizzy every once in a while and slight headaches. My period is also late by a little over a week, but when i took a test 3 days ago it was negative. I really need to find out what is going on with me! I can't take this anymore. Please help.