What Causes Delay In Menstruation Other Than Pregnancy?

i have been experiancing nausea and headaches for over a week. i have taken several pregnancy tests they all come out negative. i went to gyno yesterday no answers about what it could be. i am worried i feel awful and it is hard to function in my daily routine. last night was the first time i also vomited. wehat could it be. i have also had sinus drainage for about two-three weeks, could this have something to do with it?