4wks ultrasound

I went to the OB I wasn't sure how far long I was according to my period I should have been 7wks. They done blood work and a transvaginal ultrasound and only saw the black little sac, and the doctor then asked did we do our own pregnancy test she said yes and HCG levels are 925. He said those measured to be around 3-4 weeks. I am thinking I am more of 4 weeks I've heard U can not see even a sac at 3 weeks. & his assistant said my levels were great. He took pictures of my ovaries and checked my Fallopian tubes and said everything looked good and said my uterus was thickening good and set me to come back at March 2 for another ultrasound. He didn't seemed worried at all. I'm a first time mom and just wondering if all of this is normal. Going to be hard waiting this long to see another ultrasound. Prayers are appreciated.