Missing My Period

I've been taking the birth control pills for about 6 months, and had normally my period, i stopped taking the pills last month in 6 october, i had my period in the 9th of october. It was expected that i have my period in the 9th of november. It's the 25th of november and there is no signs and i havent had my period yet. I did the pregnancy test and it was negative. I've missed many pills in the past 6 months i was taking the pills, but never been pregnant. Im very worried and i need to know what is changing?
Thank You