What Causes Spotting With No Regular Menstrual Period While On Birth Control?

I started taking birth control six months ago and everything has been normal up until this month. I regularily take the first twenty three pills and then have my menstrual cycle for the last four to five days of the package. I finished a package last month and started a new one. I take it regularily, 8pm, every night, never skip a beat. Around day thirteen, I started to experience very mild light bleeding that seemed slightly off colour from regular menstrual cycles. It was very light for about eight days altogether and then stopped, two days before the seven sugar pills at the end of the package. Now I am due to start a new month of birth control the day after tomorrow and still no menstrual flow. I am worried, but I was wondering if there is anything else, (other then the dreaded baby thing..) that might be causing this?