First off I am only 15 years old. And my period is VERY irregular. I am sexually active, the last time I had sex without using protection was December 4th so it was just a little over a month ago. My last period was 2 months ago. Some people say it's old blood, but this hasn't ever happened before, so can someone please tell me what this could be?

http://www.momswhothink.com/womens-health/brown-vaginal-discharge.html

The link above describes what it could be. If it goes on for too long, you should see a doctor.