Measuring 2-2.5 smaller

Hi we are in uk and paid for a private scan at what i thought was 7 weeks 4days the did internal scan and saw gestational sac measuring 23mm and yolk sac they said i am measuring 5-5.5 weeks and asked me to go back in on 26th for re scan i am so scared has anyone measured alot smaller i got my first very faint positive on 12th september and continued testing for a week which all got darker i have had no bleeding/spotting or pains tia