brown blood before period??

Natalie • Wife | Teacher | Mama | Small Business Owner | Coach
Been on BC for a month, finishing my first month this week. Had unprotected sex earlier this week.. not sure if bf had precum but says didn't cum inside me. I take my pills pretty regularly. Not on the dot though. If I miss one I take it the next morning. Supposed to start period in a few days.. 5-6... roughly.. 
having normal pre-period cramps no new cramps in a different place. So backache mostly. But this is the third day I've had brown blood. It's not light enough for spotting.. almost covers a regular sized tampon..  but I usually have some sort of spotting this dark brown color before period anyways.. but it doesn't last for three days... usually just day before. Is this a light period, implantation bleeding, or what could be the cause? Any advice? 
Help a girl out please!