Dark brown spots up to now, pregnant?

i am 22 years old an i had a sex with my boyfriend last Dec. 16-17. we did not use any kind of contraceptives. my menstrual period was expected to occur on the 22nd of the month and yet i got my period during the 21st of the month. i am not very regular with my period dates that's why i am not so bothered having it earlier or later than the expected date. but then, my period seems like just a light bleeding and it stopped on the 4th day of my cycle, which is not normal for me cos i usually have my period for a week. so in the fifth day of my cycle, i took a contraceptive pill and i missed taking the pill on the 7th day and i hurriedly took 2 pills on the next day after knowing that i have missed one. the next day, i have found some brown spots on my underwear that made me so nervous. and i still have some dark brown spots up to now. am i pregnant? cos i really can't be pregnant now! please help me..