How did you get the number of 5 weeks 6 days pregnant? If you got the estimate 5 weeks 6 days (as of March 25) from an ultrasound, I'd say that unless you had sex with someone after the 21st, the baby probably came from the sex on the 21st. But you probably didn't conceive that day. Your ultrasound (if that is what gave you the count) suggests a conception date around February 27. Sperm can live a while in your system, and a 5 w 6 d number on March 25 suggests you probably you didn't ovulate until at least around the 25th or 26th. That would be when you conceived, only when you ovulated.
Your last period is when it started not ending. You said your last period was feb 13th and you had sex on the 14th doesn't make any sense unless you had sex on your period the next day. I think you conceived around the 21st.
Right after your period I'm guessing.