What Causes Brownish Vaginal Spotting Before The Periods?

I had unprotected sex the day before my period and came on as normal the following morning. My next period is not due for at least a week and a half, but yesterday I started having light bleeding/spotting, which is brown in colour, accompanied by period pains. I have never had bleeding between periods before and my period has never come early. Could this be implantation bleeding, even though I have no had sex since the day before my last period? If not, what could this be?