Could I be pregnant?

I got my last period on July 28th. I have a 28 day cycle - right on the dot. I had unprotected sex on August 9th but only for a little bit , meaning he put a condom on the second half. Yesterday I had light brown spotting when I peed. I checked on line and the 9th I was fertile and the 20th (yesterday) was the implantation date. I'm so confused.