Using a 15th-week ultrasound to try to determine when a baby was conceived is pretty iffy, because as a pregnancy progresses, babies can develop more quickly or slowly than average, and due-date calculations are based only on averages. If your doc had told you on week 7 (counted from the first day of your last period) that the baby is due March 25, you could assume the baby was conceived right around July 2. But since your first ultrasound was not until well into your second trimester, you have to add a margin for error of +/- 12-15 days to the due date (and therefore to the implied conception date). Even if you do that, however, the guy from June 17 is not sounding like the dad. You had a period on the 30th, after all.
Pregnancy from conception to full term takes 38 weeks. However, the time period doctors use to count out a pregnancy is 40 weeks. The time period doesn't begin on the assumed day of conception, it begins on the first day of your last period, or on a calculated "first day of your last period" if you have irregular cycles or your first ultrasound was late.
This means that the first two weeks of this assigned 'pregnancy' time period, you haven't actually conceived yet.
It's done this way because before ultrasounds, the only signal a doctor had to start the pregnancy count was the first day of the last period ... a period is obvious and conception is hidden. Even though we have ultrasounds now, the medical profession has stayed with this counting system because all the research is calibrated to a 40-week count.
Maybe this will help you. I've posted it many times on this forum but you might not have seen it.
Women who just can't relax about having had sex that is obviously in the wrong time period to produce the pregnancy (such as, before her last period) might be dealing mostly with anxiety and not really with worry about paternity. The anxiety can have as many causes as there are women. It could be guilt over how she behaved, especially if cheating was involved, or a concern that the fact she did it might mean she is not actually happy in her relationship. It could be catastrophizing, such as, if he finds out she cheated the world will explode. It could be worry about being a good parent, or wishing the other guy was the dad and being ashamed of it, or resentment of her husband, or fear of childbirth, or fearing divine punishment for being a single mother, or not really wanting to be a mom, or worry about money, or something else entirely. Worries like that are not easy to control, especially if the problem is shame or guilt. (This latter kind of lingering angst often is a specialty of women who have never had to face up to having done something 'wrong' that can't be taken back, glossed, or fixed.)
When such a feeling drives anxiety, the brain hates that it doesn't feel solvable. A person's mind in this kind of existential stress often lets the anxiety settle on something cut-and-dried that to the brain feels more controllable. (Like obsessing over what if the medical evidence is wrong and the father is someone else, what-if what-if what-if what-of.) Unfortunately, no amount of rational answers about paternity will solve worries that aren't actually, deep down, about paternity. Talk to a counselor about your bigger fears, and the obsessive worry about paternity will begin to fade.