I have the nexplanon implant and had it replaced after 3 years this may. during the replacement the doctor switched the implant from my left arm to the right. 3 months later i'm having dark brown discharge for 1 month and now heavy bleeding. help?

This is very normal. Nexplanon is notorious for irregular spotting and breakthrough bleeding, even if you did not experience this during your first implant. Wait it out a couple more months to let your body adjust, and if the bleeding continues and/or is prohibitive to your quality of life, discuss with your provider about removing it and switching to an alternative form of birth control.