Here is some information. You probably know most of it, but just in case, I'll lay it out for you and for some other person who might come next with the same kind of worries and read this.
1) Guilt or anxiety can make it hard to even understand, or react normally to, scientific and medical evidence in the logical way you might if you weren't distressed.
2) A certain number of weeks "pregnant" told to you by a doctor, ultrasound tech or nurse means that number of weeks since the first day of your last period, not the number of weeks since conception. If a doctor looks on an ultrasound and says you are 5 weeks "along" or 5 weeks "gestational age," or 5 weeks "pregnant," he is saying the baby is of a size consistent with conception being 3 weeks ago. The pregnancy time period is counted by the medical establishment as beginning on day 1 of your period, with the assumption that conception would happen two weeks later.
3) Having a period come means you are not pregnant.
4) If your period comes two days after sex, it means you are not pregnant, and also that you were not ovulating when you had the sex. (You ovulate two weeks before your period, not two days before your period.)
5) Sperm in your body can live 4-6 days, and then it dies. (There is even disagreement about whether it is strong enough by the 6th day to penetrate an egg.) Sperm does not lurk in your body for weeks and then suddenly impregnate you. Unless the prior sexual event was less than a week ago, its sperm is gone.
6) Unless you know from tests that a man is infertile, assume he is fertile. Having slept with someone for a number of years and not gotten pregnant does not mean it is impossible to get pregnant. Every seminal ejaculation produces millions of sperm, and even people with a lower sperm count still have a number of sperm. It only takes one.
7) If you can't break away from obsessive worrying about paternity even in the face of all the medical facts showing who is the dad, see a therapist. Anxiety is its own problem, don't let it settle on paternity if your data don't give you a reason to question paternity. You can waste your whole pregnancy (and bore and frustrate your friends and trusted confidantes) by being uncertain when there is no reason for it. Look for the real reason for your anxiety and address it, and unjustified fears about paternity will fade away.
It sounds from your ultrasounds like you got pregnant around March 4, as you would expect from your ovulation date.
Did they give you a due date at that very first ultrasound on March 25?