Could this be implantation bleeding?

I've been having unprotected sex this month with my husband. My period was due 4 days ago. I've been having brown spotting for 3 days which has got lighter each day, only enough for a panty liner. It seems to have stopped this evening. I haven't had any period cramping or sore boobs like I get before my period. Just mild back ache. Do you think this was just a weird period this month or implantation? Its come when my period should which is why I'm confused. I will take a test ASAP but need to wait until I can get to the shop.