Who is the father, when did i get pregnant

Patient: “i had unprotected sex jan. 13th . my period came normal in the beginning of feb. like feb 2nd. then i had unprotected sex on feb. 9th as i was at the ending stage of my period with someone else. my period was late in march . i went to the doctor they told me i was 5 and a half weeks pregnant , when did i get pregnant?