Pregnancy :: Conception Date Doesn't Match Due Date

I am pregnant about to be in my 7th month. I have been confused for some time however, about my due date not adding up to my conception date.
I wasn't having intercourse for months and one night with a friend I had protected and unfinished sex may 11th. Unexpected and unsatisfying. I then had unprotected sex the next weekend on the 17th with another man that I have been seeing on and off for years now that was finished and satisfying and he did ejaculate inside of me. I found out 2 weeks later that I was pregnant. I told both men but I was more than sure that the second man was the father considering unprotected vs protected. However I still knew it was a very slim possibility. Looking back at my cycle at the time I also believed I was ovulating the weekend of the 17th. But I could not give a good estimation of my last period. I just know it was in the beginning of the month.
My first ultrasound in the room the tech told me how far along I was (11 weeks) which added up perfectly from who I absolutely was sure it was the 17th. However the doctor I saw after the ultrasound I assume went off of growth of the baby or off of a possible inaccurate LMP estimate. But they told me I was 12 weeks which was a week ahead which would add up to the first guy on the 11th. Then proceeded to tell me my due date was on the 1st of February. But according to my thought of conception date (17th of may), my estimated due date should fall around the 7th-10th depending on my cycle which I can't remember exactly for the life of me.

I would really appreciate any kind of help at this point. And if the best answer is to get a paternity test. That can be done. Thanks again for your time.