TMI Implantation Bleeding or Period?

I’ve had unprotected sex with my boyfriend last week and about 20 hours after I took a plan B. My period is due tomorrow but the bleeding started Wednesday and at first it was a dark brown color. I’ve been experiencing bad cramps on the first day and normal PMS but I’m still nervous that this is implantation bleeding because my flow is lighter than it normally is. Could anyone please tell me whether this looks like implantation bleeding or a regular period that came a few days early. Thanks in advance.