Thanks Doc, I Had Unprotected Sex With My Gal Friend

thanks doc, I had unprotected sex with my gal friend on june 20th through the night till 21stsunday, she had her period on the 27th of june but only for three days, she normally gets her period for 5 days,, she had a day menstration the end of july and that was it, we did a pregnancy test and it is showing positive,i am wondering if I could have caused it please educate me if I am definitely responsible