Pregnancy of Birth control?

I'm 3mnths postpartum on the 7th, my period is 8 days late. The 12th of August I got my nexplanon implant. I bled extremely heavy for 14 days straight. Finally it left and now my period is late. I won't lie, my partner and I do not use protective sex and haven't for awhile hence our first kid not long ago... is there a chance I could be pregnant on Nexplanon, I've read it's very rare but I want to know if I should take that extra step to make sure.