further along then I thought ??

I had my period July 23rd -27th and our wedding was July 30th and we assumed the baby was conceived on the 31st of July . But when I went to the doctors they gave me 12 weeks 2 days n I thought I was only 11 now today I'm 13 . But when I went for a second ultra sound the other doctor said that baby looks 14 weeks . I honestly don't remember doing anything before the wedding because we were trying to hold off . This is freaking me out is my baby just big or is it possible to be pregnant well on my period ?