implantation bleeding or period?

Okay... I started my last period July 17th. I got off the 22nd. My periods are usually 28-30 days apart. I had unprotected sex 3 times, August 2nd, 3rd & 4th. I started bleeding lightly August 8th & it lasted 3 days, my periods usually last 6 days. It was Not like my normal period at all. I usually Cramp really bad & have to load up on Midol. But I only cramped on one side just a little, nothing like i usually do. It was a lighter flow (I usually bleed like crazy.. like have to change every hour or so) & a lighter color than usual. This has really puzzled me, because this has never happened before. Please help!