First ultrasound....concerns

So yesterday I had my first ultrasound. I found out I was preganant May 16th with 3 positive in home test. I went in Monday the 18th and found out I was 6 weeks. Well I scheduled my ultrasound yesterday the 27th since I'm 7 weeks. I was very excited until I got the results. It was a transvaginal ultrasound and 30 minutes later and 4 nurses/doctors later they finally located my uterus and the amniotic sac. Well there was nothing visible inside the sac just black. Well when they measured it, it states I'm 4 weeks and 3 days. They said that means I got pregnant about 2 weeks ago putting me around the 13th but how could I have gotten positive tests in such short time. The other concern was that it may not be a viable pregnancy which scares me. I don't know what to think. I looked up how small the embryo is at 4 weeks and they said the size of a tomato seed, is that why it wasn't visible? I don't know what to think. Any ideas? Advice? Also due to the ultrasound and the long time it was inside me I'm in a lot of that normal?