10 Days late on period but Negative Pregnancy year!

My last period was on 12/21/2018 I was due to start on the 28th on this month (January) but I have yet to start yet I’ve taking 3 pregnancy test and they have all been negative!

I’m so confused on why I haven’t started my period yet... am I pregnant and if not than why haven’t I started yet!?

Any help would be appreciated! Thank you for your time! 😊