Is A Heavy Bleeding Period, Cramping, Back And Umbilical Pain A Miscarriage?

Hi I'm on implanon, which is due for replacement (it's been 3yrs and 4mth). Last week I had my period which lasted for 4days, and during those 4 days I was bleeding quite heavily which I never experienced since I been on implanon. Now,4 days after my period finished I am experiencing an extreme cramp, like I'm having a very had period pain. On my lower back and around my belly button. What's happening to me? Could it b that I had a miscarriage last week? I must also put that I am very fertile, I got pregnant straight away with my second child straight after I took my implanon off. Please help