Help negative pregnancy test bit still no period after three weeks

Brittany • Hi I`m brittany I`m 18 and I`m studying childcare I live work with children and can`t wait to have my own. And also I run a page on facebook called helping hands in Christchurch New Zealand
I'm two nearly three weeks late for my period. I have taken over 6 test all came back negative but yet I have been going to the toilet more and feeling sick throughout the day also have been eating alot more and having cravings for sweet foods also have been more tired than usual. Any ideas? I need help I want to know what I could be