What Causes Delay In Menses Other Than Pregnancy?

My first day of my last mentral cycle was 12/25/11. I still have not started my period. Early in the month, i had a sinus infection and bronchitis. I was put on an antibodic and zpack. I then quit smoking 5 days later and started using the nicotine patch. Could this cause the delay in my cycle? I've taken a pregnancy test and it came back negative but my urine was very clear.