Keep in mind that even when done correctly, the withdrawal method is not 100% foolproof. Many babies have been born from that method. Based on your last period, your fertility window was most likely between March 11th and March 21st. The encounter from the 10th may have caused a pregnancy. Unfortunately, you will not know until three to four weeks from now, when you either get or miss your period. If you are not interested in a pregnancy right now, I highly suggest another form of protection.