[Rivière]

Each person's period varies as there are certain things that have to be taken into account such as any medical problems or stress which can delay a period or cause it to arrive earlier than expected. Usually periods arrive around once a month in a healthy female although periods aren't always so regular and may not be exactly 1 month between the next. (e.g 27th November), this number can give or take a few days to a few weeks.

What I would suggest is that if you're trying to make a good and reasonably accurate time for when your period arrives, note the date that it arrives each time and from there you can help yourself determine when your next one should be.

If as you say, your period arrived on the 27th, you may even skip the entire of November and start at the beginning of December. This is completely normal as is having 2 periods in the same month.