Period came on 17th went off the 21st had sex the 21st. Layed still 15min after. I'm trying to conceive?

You are way off. your fertile days even you have short menstrual cycle. " Laying still 15 min. after " may facilitate the conception but if you do it around your ovulation time. This ovulation time falls roughly in the middle of your menstrual cycle provided that your menstrual cycles are regular.
You ovulate. 14 days before your next period About mid cycle. Have sex about every other day around that time for your best chance to conceive.