you could very well be a full month, actually most likely that you are. Ask some girls on here, they'll tell you that you can get many negatives before a positive. If your last period was Nov, I would go by that, but the doc will give you a EDD depending on either that or if he gives you an U/S, he'll probably go by that.

Without knowing exactly when your last period was, you will need to find out from your doctor. A dating scan will be able to specify a due date for you and give an estimate of your last period.

Congrats!