Unsure

My period is supposed to come this week, usually it's here by today but it has not come. I often get a small amount of old blood at the start which lets me know my period is coming but I haven't had that either! I've had stomach and back cramps but still nothing. I had unprotected sex twice in the last week and a half so I have thought maybe I'm pregnant but I have done 2 home pregnancy tests and they were both negative. Any ideas what could be the reason?