Hi, I have been on the birth control tri-sprintec for months. Recently, my last period did not want to start at all, it was several days late and only lasted about 2 days until i started my new pack of pills. I am now into my second week of this new pack and dark red/brown spotting is occuring. I have never had this happen before and I've been using birth control for 3 years. What is happening? I am 21, 145 pounds, no major medical issues, although I am on an extremely low dose of doxycyclen for acne.