Late cycle.

I have NEVER had a late period ONLY with my 2 pregnancies and I knew the first day of my missed period. I had a miscarriage back in the spring early summer and I have had it right in time the past 5-6 months. I was due Christmas Day to get my period and I took tests yesterday and got a invalid and a negative. Two different kinds of tests. I still have not got it today when it said give it a couple days, even had sex yesterday and not even spotting or anything of a sign of my period.... How certain can I be that I am pregnant... ?