Three Weeks Ago I Had A Very Heavy Period, And

three weeks ago i had a very heavy period, and a week after it was gone i started getting a bloody discharge for three weeks straight. It went away for 4 weeks and instead of getting my period i got this bloody discharge again. I am fifteen and i got my period last year. I have never had sex and i am quite worried. What should i do? What is wrong with me? Is this common?