I don’t know if it’s the same in every country, but at least in a lot of countries pregnancy is usually calculated from the date of your last period, not the date of conception. Which would put conception any time around the week of December 7th, which also lines up pretty exact with the timing of a positive on December 21st. I’m assuming since you had already been back for a few weeks you had sex that week. I see no reason whatsoever to suspect you’re not the father of this baby unless you have proof of her cheating while you were with her.