What Causes Brown Discharge Even While Taking Birth Control Pills?

I'm 15 almost 16, and I am sexually active. I am on Birth Control and have been for a year now, I was on Lessina and now its Lutera and I have been taking that for like 4 or 5 months. At first my Urine has just been smelling horrid, and then the next day, I am having like brown discharge. I am thinking its old blood but I am not sure. When I wipe its like a reddish tint.. and its all that color. I have also been having like abdominal pain.. I have no idea whats wrong and It is really scaring me. Do you have any Idea what it could be?