Is Pregnancy Possible If Intercourse Was On The Last Day Of Menstruation?

I had unpretected sex several times on the last day of my period about 28 days ago and i have taken several pregnacy test every since i took one today which is suppose to be the start of my period and still got a negative but no period. should i asume im preg. and go to the doctors for a real test or should i wait. could this be caused because i missed 2 weeks of my birth control last month and it is just throwing my period off?