So my last period was between December 19th and 22nd of 2014 I'm pregnant and supposed to be due September 28th which I've been told I conceived between the 1st and the 5th of January. I had sex with my partner December 31st January 1st and the 3rd and 4th as well.
Well this month on Tuesday August 18th I had an ultrasound saying I was 34 weeks and 2 days.
What day did I conceive? And is my due date wrong?

Its literally IMPOSSIBLE to give a specific day... the best they can do is make a guess that will be accurate to only plus or minus 2 weeks. If you slept with more than one person during that month... you will have to have a DNA test done upon the child's birth to determine who the father was.

Your dates all match up within a normal time frame for conception and due date.

I mean no disrespect, is there a concern over sex with someone else? Why are you concerned over a specific date?

There is no way to know a exact day. Even a exact week is very hard to be sure. Conception dates are estimates

Calculating conception an due dates is an inexact science. It is your doctor's best guess but can be as much as two weeks off. So, unless there is an issue of paternity, what does it matter the exact date?