First ultrasound 7w4d

Went to a free clinic today to get my first ultrasound and the tech couldnt see anything they said it was empty but it could be way to early . he measured the sac and said it was 1.4cm so about the size of a dime cut in half and that it was measuring 6w4d instead of 7w4d . he said it was probably just to early to see anything and that i should come back in 3 weeks . even though he reassured me that this is common im still freaking out because i expected to see a some thing anything at all . Anyone else go though this? How did it go ? Any and all advice is welcome