How To Evaluate The Conceived Date?

I had sex on December 31st with someone ( other than my bf). That was the first day of my period also. Early February I had a positive pregnancy test ( 10 of them). On February 11 I start bleeding and on February 13 I eliminate a big clot with tissue and grey dots. I had also some pains. After that ,all my pregnancy symptoms were gone and my pregnancy test showed negative .I was bleeding for about 10 days. I thought it was a miscarriage…On February 15 I went to a sonogram and the doctor said that my uterus is clean and there is nothing inside, but they couldn’t confirm my miscarriage since they didn’t see me before. After 5 weeks I had my period ( March 17), and then on April 22nd I found out that I was pregnant . It s true since March me and my bf are trying to have a baby . My bf has lazy sperm. I went to a sonogram on April 29 and the doctor said that I am around 4 weeks pregnant . My question is if this pregnancy could be the same from the December 31st sex ? Is it possible? Is it possible that at the sonogram from February the doctor couldn t see the sac cause was too small? Is it possible that this sonogram from April to show a pregnancy if 4 weeks but to be pregnant in 2 months? I know very well the process of conception, sperm, ovulation, fertilization but I am still concern... I know what I did was wrong , I got my lesson but I am truly scared. I tried before with my bf to have a baby for past 1.5 years and nothing happened... It is true that we were very stressed in that time and now for the past 6 months the stress is gone and boom..now I am pregnant. Please give me an advice. Since December nothing happened with anybody else, just then, once…the mistake of my life... I am really worried. I know that the period can be over the pregnancy and I am truly scared. I can’t rest, I am very very stressed and I can’t enjoy my actually pregnancy. I know that the ultrasound is accurate but I am terrified …maybe is the guilt….Please give me your opinion… All the ultrasounds that I had : April 29 – showed 4 weeks pregnant May 28 –showed an 8 weeks fetus June 9 – showed 10 weeks and 4 days pregnant June 18- showed 12 weeks pregnant CRL 5.44 cm June 24- showed 13 weeks pregnant, CRL 6.4 cm, 159 BPM July 14- showed a 15w and 5d pregnant July 22 – showed a 17 weeks pregnant, CRL 11.5 cm I have my due date on December 30th 2014.Please help ! Is it really possible the ultrasound to be so many weeks off?Is it possible that this baby to be conceived on December 31st 2014.