How's early is too early to see anything on a Vaginal Ultrasound?

According to my LMP I should be about 5wks 2days.  Since I've had two miscarriages they asked me to come in immediately to start monitoring me. When they went in with the Vaginal Sonogram they saw no sac, the nurse said she saw the thickening of my uterine fluid and a cyst.  She said both were good signs and even pointed out where she thought the sac would start to form.  In the end she said it was too early to tell anything definitive.  They scheduled me for another check up next week. I know I could've ovulated later than I thought. Idk. I guess I'm writing this to hear other stories that may help ease my worrying?