Ultrasound.

Hi mummies! 19th Nov, I went for my ultrasound at KK Hospital. I saw one tiny sac. 20th Nov, I went for another ultrasound at Mount E Novena for reassurance.. but this time the tiny sac seem invisible. Mount E Novena gynae told me to come in two weeks time for another ultrasound. I am so so worried. Any mummies have such experience before? My last period was on 15th October. Is it too early for sac detection? ?