All tests are negative, still no period

My period was due right around Christmas. It comes like clockwork each month. I've had some home tests that appeared positive and then everything has appeared totally negative since the day before my period was due and all symptoms I was feeling disappeared. My period is now 10 days late and still no sign of it. I had HCG bloodwork done and negative for pregnancy. Has anyone experienced this before?? I'm supposed to have an ultrasound Thursday and then discuss inducing my period.