What Causes Brown Spotting During Periods?

hi i am 21 years old My ill time start in january 27-1-2010 and finished in 31-1-2010 always as this but i was taking pills the 5 day of my ill time i am married 1 year and in 9-2-2010 february i stoped pills i am useing Yasmin film coated tablets i stop cz i want to have baby so later 1 week i saw dark brown spot one day and stop later pink brown spot and stop again and in 16-2-2010 february my ill time came and in 22-2-2010 finished later i did test pregnancy in 7-3-2010 it was negative , i am worried it there is a problem when i can do test again and can i get pragnent cz i am worry if my illtime came this month i will feel so bad. thx so much for helping.