What Causes Delay In Period?

Hi, I was injected with Kenalog in my scalp for a skin disease that I have. A week later I had bleeding two weeks before my expected start date.The last day of my last period was January 23,2011. Today is March 2,2011. I am obviously over a week late. When I called to ask my dermatologist if the Kenalog can effect my menstrual cycle, she relied with a no. I have taken pregnancy tests and they come back negative. What is wrong with me?