What Causes Spotting And Delay In Periods?

Hi. I was supposed to get my period yesterday. I have been spotting (which is dark brown with a funny smell) for a week now. I just finished my pack of birth control (which i wasnt taking on time and at times I would forget to take it but always take it when i remembered). I have some cramping but still no full blown period. What could that be? (I did have unprotected sex with my fiance)