Period then bleeding

Arielle • 20 yrs old married to my best friend who I known with for 6 years we had miscarriage in Jan 13 2016 now trying again (: we delivered our beautiful baby boy March 22 2017

Well started my period nov 29 and last til dec 5 is stoped and dec 10 I started bleeding again like pretty heavy . Does anyone know why or has this happen to them

Btw I’m on the nexplanton (implant in the arm) had it for 8 months now