If you had a normal period on 13 December then you likely were NOT pregnant from having sex on 2 December.
With a period starting on 13/12/2016 you'd be expect to ovulate between 24 and 29 December. The egg can survive up to about 48 hours so having sex on 1 Jan, depending upon when you ovulated exactly, pregnancy is possible. Using protection obviously greatly reduces the chances.
No one, not your doctor, no one can tell you when you ovulated after the fact.
The estimated due date corresponds to a LMP starting on 14 December 2016. Dates are normally calculated by adding 40 weeks to your known LMP. Ultrasound dates are also just an estimate.
From your post, while EX-2 is a remote possibility, your husband is the most likely father.
Not being regular makes it difficult to calculate these results. They are based on a 28 day cycle. You will need a DNA test to confirm. You can do it now. Google "non-invasive" DNA test.