also cant remember my lmp
Everything you have said here from your ultrasound evidence points to conception around August 18-22, and this time slot is also supported by your baby's birth measurements. Nothing in your medical information suggests conception on July 28. Even if it did (which it doesn't), sperm does not last 10 days in a woman's body.
The medical count of weeks in a woman's pregnancy time period begins on the first day of her last period, not at conception. The first two weeks of a woman's pregnancy time period are the weeks between the period and conception, when the woman technically is not even pregnant yet. But doctors begin the count with the first day of the last period even so, because it is an obvious signal. They are not saying you got pregnant then, you were having a period! They just use it to start the count.
(If a woman doesn't know the first day of her last period, the ultrasound computer will measure the embryo and assign one, to start the count. It gets this from the information in its database about embryo size at a given number of weeks.)
All counts of "weeks pregnant" given by a medical person include this two weeks margin at the front end. You ask about July 28 as if it means something, but it doesn't. Your ultrasounds say you conceived in mid-August and so does your baby's size at birth.
Well, "there is nothing to worry about" depends on if it is the July guy or the August guy you are worried about, which you haven't mentioned. If you are worried about the guy in July, don't be. He was at least a month too early to have produced a baby born on May 8, possibly even a month plus a week or two too early if the doctor's count of your baby's development at birth was 100% accurate. But your early ultrasounds make it clear you did not conceive from the sex on July 18. They would have given you counts that were four weeks different than they were.