Hello Cam. You poor thing! What an experience! Did you have an internal scan? It would have been too early for anything to have been seen with an external ultrasound. Even with an internal, it may well have been too early for a yolk sac to have been spotted. And maybe even a gestational sac. I'm sure there would be people on BB who would be surprised that you were sent for an ultrasound at 5w.
Anyway, I've found a website for UF MED 1st trimester ultrasound scanning (google it and you should get it. I'd PM it to you but you don't have enough posts yet for me to be able to do that) and it had some information that you would find relevant - and hopefully reassuring.
The gestational sac is the earliest sonographic finding in pregnancy. The gestational sac appears as an echogenic (bright echoes) ring surrounding a sonolucent (clear) center. The gestational sac does not correspond to specific anatomic structures, but is an ultrasonic finding characteristic of early pregnancy. The gestational sac first appears at about 4 weeks gestational age, and grows at a rate of about 1 mm a day through the 9th week of pregnancy. Your ability to identify an early gestational sac will depend on many factors, including the capabilities of the ultrasound equipment, your approach (vaginal or abdominal), your experience, the orientation of the uterus (generally it is easier to see if the uterus is anteflexed or retroflexed), and the presence of such complicating factors as fibroid tumors of the uterus. While a gestational sac is sometimes seen as early as during the 4th week of gestation, it may not be seen until the end of the 5th week, when the serum HCG levels have risen to 1000-1500 mIU. As the pregnancy advances, the next structure to become visible to ultrasound is the yolk sac. This is a round, sonolucent structure with a bright rim. The yolk sac first appears during the fifth week of pregnancy and grows to be no larger than 6 mm. Fetal Pole - A mass of fetal cells, separate from the yolk sac, first becomes apparent on transvaginal ultrasound just after the 6th week of gestation. This mass of cells is known as the fetal pole. It is the fetus in its somite stage. Usually you can identify rhythmic fetal cardiac movement within the fetal pole, although it may need to grow several mm before this is apparent. The fetal pole grows at a rate of about 1 mm a day, starting at the 6th week of gestational age. Thus, a simple way to "date" an early pregnancy is to add the length of the fetus (in mm) to 6 weeks. Using this method, a fetal pole measuring 5 mm would have a gestational age of 6 weeks and 5 days.
Were you asked to come back for another scan? And a follow up blood test? Good luck with your pregnancy and please let us know how you get on. Personally, I'd be changing doctors!