Period or Implantation?

My cycle is normally 37 days with a 7 day period, my period was supposed to start 5-16-14, but there was only spotting, it got a little heavier since then, but it's not heavy enough to be my normal period (it's usually medium-heavy and requires 2-4 pads a day.) the way it's bleeding right now it would only require a panty liner or one pad a day... I had unprotected sex 3 weeks ago. Help please. :(