What Causes Menstruation Twice A Month At The Age Of 17?

Hi. I amk 17 years old. I weigh 117 and I am 5'1. I am on Ortho Tri Cyclen and have been on it for about a year and a half now. So, I recently went off of my period about 2 weeks ago, and now i'm on it again. It started off really light and it had a dark red/ brownish color. Now, it's really heavy. I'm suppose to be going off of it tomorrow, but it doesn't look like I will be. Please help me and tell me if this is normal or not.