confused. u

Ladies I'm 20 years old. eve told me that I was supposed to start my period on August 13 yet I haven't started. the last two months I been starting around the 16th or 18th. Now it's already passed the 16th and I haven't started. I have taken 2 pregnancy tests but at night cuz some people told me to try that that but both came back negative. Could use some helpful information what could be wrong. I haven't had much symptoms for my period like a lot of the time I start cramping a week before I start and I didn't.