Thank you for posting your question. I will do my best to help you.
I have tried to understand your query to my best
Increased Physical acitivity ,change in diet do have minimal role in disruption of Menstrual cycles .(It is not abnormal)
Birth pill also have impact on cycles & bleeding .
All theses above are not significant if changes are minimal. Period can come a week early or a week later.
If associated with Headaches
which are constant & severe, or any history severe bleeding, then need to consult your Gynaecologist at the earliest for further advise.
Otherwise, As long as your taking the Birth pill regularly, need not be worried(however all contraceptives have percentage of failure which one has to keep in mind,& need to check with Gynecologist if at all any missed period )
I hope that I have answered your questions fully and been of some help.
If you have anymore questions please contact me again anytime