Implantation bleeding

When does implantation bleeding occur? I had sex 4 weeks ago and I got dark brown bleeding for 6 days 5 days after sex. It was a light flow. Yesterday (7 days before eve said I was due) I started bleeding super heavy and bleeding through supers (which is normal for me) and I passed clots but it is starting to slow down and I'm just scared. Could it be that or is it just a period?? Ive gotten major cramps (minor ones throughout the 4 weeks also) but Ive also been nauseous since I had sex. Im not on any birth control. please help!!