Question about ultrasound dating?

hi all this is a very hard question to ask u all but ur all strangers and icant talk to anyone in my family about this personal issue. i havea3 week old daughter and i was wondering about possible paternity. i was with my boyfriend multiple timesfor the month of july and august...my periods were ALWAYS on time like clockwork....due to my last period being july 25th it said i was due april 30th. well i was also d on the 17th of august. ok so anyway when i went to the ob gyn they looked at that little due date calculator wheel and said due to when i had my last period i should be due april 30th. well i had an ultrasound at 8-9 weeks and it said i was due may 13th (which put me exactly to the guy who d me) i was just wondering if the ultrasoundcouldve been off at ALL? i had my baby 3 weeks early on april 25 (remember my lmp duedatewas april 30th). is it likely it was my boyfriends all alongf and trhe ultrasound dating was wrong? im soo ashamed about this n no1 knows i figured id know whos it was when the baby came out shes light...the guy was like a tan and my bf is a DARK skin puertorican...could it still be his? sry for long post!! all help appreciated