I Started Spotting 8 Days Before My Period And I

I started spotting 8 days before my period and I am not sure it is implantation bleeding. I have been spotting for 4 days and today it is heavy enough that I need a pad. I am on a low dose birth control and to my knowledge I haven't missed one this cycle. I heard that implantation bleeding cannot last more than a couple days but I am still worried. Do you think it is implantation bleeding?