Fetal cardiac activity seen but can't hear

This was big day for me to hear HB. Actually I went to doctor 5 days ago, we saw the little heart flicker but couldn't hear the sound. Today also the same thing has happened. Fetal cardiac activity is present (heart beats) but can't hear. Now I am supposed to be 7 weeks but the doctor said that according to measurement of the sac, I am 6w1d and through vaginal USG there was a veeery little sound and even she couldn't measure the HB rate. She told me to come a week later to check again. Heart flickers very clearly and visible at the screen but can't hear the sound loudly and the USG machine can not measure BPM.