has anyone had this happen before?

Can you miss a period and not be pregnant? I'm supposed to start today but I can tell just by the way I'm feeling that it isn't gonna happen today. Maybe I'm just a few days late and are over reacting but I just wanted to know if anyone has missed a period and wasn't pregnant before. I have absolutely no symptoms, boobs aren't sore, no mood swings or spotting, not bloated.