If you had sex before starting your pack it could be a possibility. However, I had finished my pack and needed to do my annual pap test before renewing my bc. I had went one week without bc until I could get it refilled. I started my period during that week and then started taking my new pack of bc. A week into it I again started my period. So I had it twice in 3 weeks. Its just the pills trying to regulate you cycle. Keep taking them and by next month it should be regular.
I would speak with your gynocologist. Any abnormal bleeding is important to bring up with them.
Definitely speak with your gynecologist. If you took a pill in that week later than usual or skipped a pill and then doubled up the next day, it could be a potential cause. Still needs to be discussed so the cause is determined.