What Causes Irregular Menstruation?

Hi, I am currently taking the Yasmin birth control pill, and have been doing so for 6 months now. 3 weeks ago, I got a yeast infection and have been taking the AZO Yeast infection tablets for 1.5-2 weeks. My period this month did not come at all and the last time I had sex was 3 weeks ago. I also did a pregnancy test which turned up negative. What is the cause of my missed period and should I be worried?