Eh, this is super common lah, a few of my mum friends who got Implanon also experienced their periods becoming irregular or disappearing completely, especially after the first few months when your body adjusts to the hormone. The thing is, everyone's body reacts differently, so while some get lighter periods, others stop altogether, which is actually fine and safe, but since you're worried about the sudden change, better to check with your doctor or the clinic where you got it, just to make sure everything's on track and rule out anything else.