I need help please!

I just recently found out I was pregnant I found out on August 17th. Went to my annual and was told I was pregnant. My daughters father and I had just gotten back together after being split up for almost a year.  I went for my ultra sound Friday sept 11th I was already 8 weeks! I need help trying to figure out when I got pregnant. I was with my ex all of July! And had just started sleeping with my daughters father in August! I feel so stupid. I just want a date and the Drs can't give me one someone please help me! I need to know if there's any ways it's my daughters fathers baby or my ex!