Due date confusion?

Katelyn
Before I had my first ultra sound I had my pregnancy confirmed at a free clinic and my LMP put me due April 18th. However after two ultra sounds they put me at two other different due dates idk what to believe. Someone explain this to me? Which is my actual due date?? 
First check up - April 18th (LMP estimate)
First ultra sound - April 15th
Second ultra sound - April 14th
But they continue to tell me my due date is April 18th even tho I've had different measurements of how many weeks/days I am. Please help.