So I'm 17, almost 18, and lately I've noticed an odorless, dark brown discharge in my underwear.  I've had this happen before, and it normally goes away fairly quickly.  However, this has lasted for over a week and a half, maybe two weeks now, and I'm starting to worry.

I'm fairly sure it isn't related to menstration or possible pregnancy because I have been on Natazia (birth control) regularly for almost 2 years now.  If you aren't familiar with it, it's a combination pill that works by stopping ovulation (along with altering the uterine lining and cervical mucus) - meaning that I haven't even had a period since I started taking it.  I have also only had sexual intercourse once (protected,) which was recently and happened AFTER the discharge had already appeared. It was starting to go away, but yesterday my boyfriend fingered me, and a large amount of it was present afterwards... I would say it was because he was kind of rough, but it still shows up on days where I don't see him.

So I'd like to say I'm not pregnant, but since I don't have a period at all, I can't use that as a sign.  Is there a chance that I might be?

Also, the discharge is all I have - no pain, odor, itching, burning, etc., and I've only had one partner, so I doubt it's an STD either.

I keep looking for answers, and I keep seeing dark brown discharge as a symptom of cancer, which is REALLY scaring me.  If anyone has any ideas as to what might be happening, please let me know.  I'd really prefer to not have to let my mother in on this, if you know what I mean.