Although it does sound like there is a connection between your taking the cranberry and your changes in your period, there really is no medical connection between the two. You are absolutely correct in taking cranberry to help with UTI's, however, as there is a known link between the intake of cranberries and the prevention/cure of UTI's.
If you continue to experience irregular cycles, you may want to speak to your doctor. While it may have been a fluke, as you mention, there could also be an underlying health problem causing the difference you experienced this month. You can also talk to your doctor about changing your pills (or the time you take the sugar pills) to readjust when you start to bleed (assuming this is something you wish to do).
If you have any questions, feel free to ask!Sincerely,Emily