Second, Worse Period?

Earlier this month I had my normal period, light flow, birth control was all on time, all was well. But now that it's the end of the month i've gotten a second period and it's the worst I've had in a while. My birth control typically control my cramps but even that can't help. I'm having to wear a pad along with a super tampon, where as I normally just go with a regular tampon. I'm bleeding so heavily that i'm exhausted and my cramps are crazy. I'm worried. Is this normal to have a second, heavier period in the same month? I'm only 16 so I may still just be irregular?