thank you for your query.
normal menstrual cycle lasts between 28- 35 days.since you mentioned you are taking regestrone
tablet,i feel it is a side effect of that and not a pregnancy
some of the other side effects include-
-abnormal uterine bleeding
such as irregular menstrual periods
-bleeding or spotting between menstrual periods
hence you need not worry for now.
however if the amenorrhoea lasts for another week,i would suggest you to get a ultrasound
done and consult a OB/GYN for further treatment.
wishing you good health