Is Brown Spotting Followed By Bleeding While On Birth Control Normal?

I am on othro tri-low birth control. I missed my pill last sunday and did not realize until i was supposed to take my pill on monday. I took both pills and kept taking my pills on my normal scheduled time. On Tuesday I noticed brown spotting and now Wednesday I am bleeding... I am sexually active and, more often than not, use a condom. Why am I bleeding?