Dark brown discharge/Abdominal Pain

Hi, I'm 15 years old and i am sexually active. I am on birth control continuously for quite awhile now and every couple of months I get blood spotting which in a couple of days turns to more of a brown colour. I take my pills at pretty much the same time everyday and i can't get this spotting to stop. About a year ago, i had chlamydia and was given anti-biotics for that, but i didnt have this spotting when i had chlamydia. Does anyone have any suggestions of what it might be? Thanks!

Tonight i have a sudden onset of pain in the lower right side of my abdomen (i have had my appendix out.) It doesn't hurt to urinate, except for pressure on the area that is in pain, but it kind of hurts to walk, the only way the pain seems to be relieved is by laying in the fetal position. From what i can tell, my abdomen isn't swollen and doesn't hurt to touch.