nervous for my ultrasound

Went to the hospital last Sunday night with cramping and a very light pink spotting one time So they did the usual work up and also an ultrasound.... they diagnosed me with a uti and dehydration but told me to follow up with my dr asap... followed up yesterday with the doctor and she wants to do another ultrasound to make sure the baby is developing normally bc all they could see was a sac on the one from the hospital... I am 6 weeks 5 days according to my lmp but the sac   measurements were a week earlier than my lmp calculations so is it possible that I am only 5 weeks 5 days or should I be prepping for they worst?