Arm implant birth control

Mariah • Mommy of 1, one day wife!
Hi, I'm 18 years old and have the implanon birth control. I've had it since Dec. 18th, 2013. And for the past 5-6 weeks I'm having a on/off period. When it's on it's really heavy and dark and I've never had that. Nor have I had it longer than 4-5 days. Has anyone else had this experience? If so what did you do?