ive heard so many stories of that happening , and everything turned out to be okay and the babies were very healthy (:
im not 19 week+ 3 days with a healthy pregnancy ;)
btw same happen with me except with only one baby. all there was was a sack and fetal pole. I went back at 8 weeks and a lil baby was beginning to form.
at 6 weeks you can't see much. at 8 weeks is when they can see more. dont worry im sure your fine. Im surprised they are not having you go back in 2 weeks rather than 1.