Who's the father? Please help

I am not in a relationship, and I had sex with 2 different guys 8 days apart.   The first guy was on April 30th and the second guy was on the 8th of May. I found out I was pregnant a couple weeks ago, and when I went to the doctor a week ago they said acording to my last period conception was on the 2nd of May, so I assumed it was the first guys because it's closer. I also used a lot of conception calendars on websites that confirmed that the 30th was the most probable date. (The 30th was also my most fertile day at 33%) while I was not ovulating at all the 8th.  Me and him got excited because I have loved him for years. Yesterday we went to have an ultrasound and the doctor said my baby looks 6.5 weeks not 7.5 weeks along like I assumed it was. And my heart sunk. That would land it as being conceived on the 9th, a day after the second guy. Could the baby just be small? The ultrasound be wrong? I just have a hard time believing I got pregnant out of my fertile window and really don't want it to be the second guys baby. I have been a mess and am in need some opinions! Thanks so much for reading!