What Causes Brown Vaginal Discharge?

I am 15 years old and no longer a virgin. It all started in August, when i had a normal period. I didn't get another period again until two months later. Since then I've gotten my period every two weeks. Right before my period comes, I've been getting clear and light brown discharges. I've done research, and nothing tells me a good answer. Is this normal, or should I see a gynechologist?