I had a period on nov 27 which lasted 6 days. in dec it started on the 17th which lasted for 9 days. i'm confused an don't know if i missed my period?

Cycles are 3-5 wks: Normal menstrual cycles are 3-5 weeks, or 21-35 days from start to start. Yours, as you describe is 21 days, perfectly normal. 5-7 days is a normal flow, although some times it may be shorter or longer.

You did not miss it: It is normal for a menstrual cycle to come every 21-35 days. If you would like better control over your menstrual cycle then consider starting birth control to help regulate them.