My last ultrasound said i'm due may 11th but i don't understand why it was change from may 14 to may 11. i only had sex on aug 3, 12, 21?

Essentially the same: Obstetric ultrasound is not that accurate that it can tell a difference reliably between three days. In my opinion the earlier ultrasound tends to be more accurate for predicting dates. Ask your doctor why they changed the due date. But essentially may 14th and may 11th are really the same time period estimate. Only 5 percent of women deliver on their "due date "