It's correct that if you had sex on the last day of bleeding of your period (and there is exposure to sperm), sperm can stay viable a few days in your body and that can sometimes be long enough for you to ovulate, especially if you have short cycles. But since you had a period on January 16 and today is January 26, it doesn't sound like you're pregnant. If what you are trying to do is rule out that particular guy, just remember that if you have sex, then have a period, it's the classic indicator that the sex before the period didn't get you pregnant.