How To Know Whether It Is An Implantation Or Menstrual Bleeding?

i have been spotting brown for 3 days...then yesterday it came heavy now today its lighter but its not as bright red and just with a few small clots. My period wasnt due till a couple days...my boyfriend and i had sex regularly (unprotected) up till last Sat when the bleeding started. Could i possibly be pregnant and such is implantation bleeding or is it menstrual bleeding?