Help! So confused!

Levikingston • ❤️Levi Kingston Lam❤️
First day of my lmp June 18. Found out I was pregnant around July. 
I went to the er around end of July cuz of bleeding. They say I was around 6 weeks pregnant. 
So in my app I'm 36 weeks and 6 days. Due date March 25. 
Today I went to the er cuz no movement in baby all day and the oulteasound lady says I'm 36 weeks and 3 days. That's mean due date is March 28 now. And baby weight around 5 lbs . Why the change?
Cuz I know for sure my lmp is the 18.