Did you ask any of the doctors or ultrasound techs specifically when conception was, or did you just ask "how far along" you were?
Here is what your story says:
Ultrasound 1, June 19 -- 5 weeks 5 days
Ultrasound 2, June 25 -- 7 weeks
Ultrasound 3, July 2 -- 6 weeks 5 days
Assuming you went to a reliable place for the ultrasounds where they were read by a doctor, not to just one of those on-the-street pregnancy test places, and assuming the pregnancy is going along as normal and is still viable, one way to explain the third ultrasound saying less than the second date, is possibly that the first two ultrasounds, they were trying to take a guess at your first day of the last period (in other words, counting the pregnancy the medical way), and the third, they were trying to guess at the conception date.
Frankly, it all sort of tends to lead to the conclusion that the co-worker and not your ex is the father, but it's not conclusive.
Ultrasounds base their estimation on measurements. Personally I've seen people come in all shapes and sizes. Just because you are still pregnant doesn't really mean that your baby is "normal" "small" or "large". The only sure way is the paternity test if that is what you are worried about. Due dates are best guesses as well. How many people do you know that actually deliver on their due date? I don't know any. I would wait until the baby is born for the safety of everyone and just do the paternity test. It will tell all of you for sure.