What does it mean when an ultrasound does not show anything at 5 weeks?

External ultrasounds can only detect pregnancy when you are more than about 5-6 weeks wereas if you have an internel ultrasound they can detect pregnancy from as early as 4weeks. In answer to your question you are either not as far gone as originally thort and would have to have a internal ultrasound to dertimine/detect pregnancy or you are just not pregnant.

Answer

I'm suprised that they wouldn't tell you when you got it done. The fetus at that age is so small its very possible that you could not see it. Or you are not as far along as you thought and it was just a yolk sack.