I started my period on march 1 and had unprotected sex on march 4 while on my period. i had unprotected sex again on march 17th, i track my bbt in an app and it said i would be ovulating on march 17th. due date is dec 6. when did i get pregnant?

You would have conceived on or about March 17 when you apparently ovulated. March 4 would have been too early to conceive. The due date calculation is imprecise and cannot be used to calculate the exact date of conception (nor does it exactly predict delivery). Based on the information provided a due date of Dec 8 would be more accurate, but that is not a significantly different.