Ultrasound gave a different due date!

Hi,

2 weeks ago i did the digital tests that said i was 4-5 weeks which matched what i thought. Blood test levels matched this timeline. my due date from all calculators and rhe doctor was feb 8th. Last night i had an ultrasound and he said i was only 5wk 3days (i thought i was 7 wks) and hes given me a due date of feb 18th. His dates don't make sense as it puts conception right near where my period was supposed to come again. Whats right?