My period is two days late... I am on birth control and always use condoms?

Hi, I have been on Ortho Tri-Cyclen for four months. I always started my period on the second sugar (green) pill since I started it. It's the end of the third day/pill now, and still no period. Before I started birth control, my periods came later. So, does this mean my body is getting used to the birth control and reverting back to the old cycle? Also, I am sexually active, and always take my birth control on time and always use a condom. What is the chance I'm pregnant?