Its obviously possible that you're pregnant if you had unprotected sex. July 28th is also the 12th day of your cycle, which is your fertile period and its different for every woman, but the 12th day could be your ovulation day, where an egg drops and waits to be fertilized. You could have fertilized this egg by having unprotected sex, so I would go to a doctor and get tested if you are really nervous about it.