Don't see why a doctor would tell you you're testing too early. That's kinda ridiculous. There are so many factors that can affect your period, like the other ladies have said already..multiple times. Stress. Weight. Birth control. Amongst other things. So if you're concerned, get down to the doctors again and tell them something's not right? Kinda just seems like you posted yesterday asking for advice and ignored everything everyone said which is a little odd.
You’ve missed a cycle, it happens sometimes. I’m sure I gave you an explanation to this yesterday with @erincharlieconor If you’re 21 days late and getting negative tests you’re very unlikely to have fallen pregnant on that cycle.
21 days and still negative? Seems like maybe you've missed ovulation. I'd just wait for my next period, if it doesn't come go back to see the doctor. Could be because of the stress of Christmas maybe?