Help!!

My last period was the last week of January for only 5 days before the 31st (don't know exactly when cause I wasn't keeping track) but that was also my first time having a period since November. I remember I had a ultra sound the last week of march on the 31st at the hospital when I was having pain and I was 6 weeks and 3 days and they told me my due date was November 21st. when I used several conception calculator tools on the internet it says I conceived on the 28th but when I count back with an actual calendar I would have been 2 weeks on the 28th. Is that right? Or did I actually get pregnant on the 28th making that the first day on?