Most women ovulate approximately 14 days before their period (give or take a few days as each woman is different). You can retest if you want but wait a few days. I would recommend First Response as being the most sensitive test on the market.
If you ovulated later than usual then there definitely is a chance. Most women don't get positives until 12 days past ovulation to 14 days past the last time you had unprotected sex. wait until you are at least the 12-14 days past the time you had sex and then take a home test first thing in the morning. Best of luck to you.