You can do it either way, lurooney.
If you don't have regular periods but have sex infrequently, you can judge by when you had sex.
Your due date is 38 weeks after having sex that got you pregnant. If you have no real memory of when your period was, but a very clear memory of when you had sex, that's actually a better estimator than going by the LMP.
Your due date is estimated from the first day of your last period,not from when you had unprotected sex
About May 22.
If you want to play around with the pregnancy calculator, it's here. You just click on the "date I conceived" button and type in the date you are interested in.
http://www.babycenter.com/pregnancy-due-date-calculator
What about august 29 2014 then what would be my due date