I think its too soon for anything bad to show up on an ultrasound (birth defect wise) that they'd be worried about (aside from there not being a baby in there) if she said not to worry about it then I surely wouldn't worry. I know its hard not to but maybe she just wants to tell you that your measuring over your date...or under your date, or maybe she wants to tell you that there's twins lol! I surely wouldn't let it bother you like crazy especially if she said "dont worry"
I think that's a good sign.
good luck to you!!