worried about dates!

I attented the early pregnancy unit on saturday morning due to spotting, i had an internal ultrasound and was told i was measuring 5-6 weeks (sac and yolk both seen) and was to return in 2 weeks to check everything was growing as it should be.....im worried as that is nearly 3 weeks earlier than i worked out by my lmp and i also had my first positive test 3 weeks ago....so scared incase the spotting is a sign that maybe the baby has stopped growing and things havent went as well as they should have 😔 this is my 3rd baby...different partner this time around but ive never experienced this before.