Ovulation is usually mid cycle. Yours is probably around day 18 to 20? But it's a guess even under the best of circumstances (or any woman trying to conceive would have a much easier time of it). Sperm can live for 5 days. It's likely you had sex past your ovulation time but there really is no safe time to have sex regarding getting pregnant. If you end up being late, take a test and you can get a positive 2.5 weeks after the sexual encounter. good luck