It would also depend on what the ultasound was for. If it were to focus say, on the ovaries looking for cysts, the tech may not pay much attention to the uterus. It IS possible if you were extremely early in pregnancy, but more than likely if they were specifically looking for pregnancy you would see a gestational sac atleast.
if your pregnant u should at least be seeing a g-sac. but it is possible not to i guess. how far along u are determines alot. i had my first ultrasound at 6 weeks and u could clearly see the sac.
Technically yes if the ultrasound was done too early. You should wait to have one done from weeks 5 and beyond, and even at 5 weeks of pregnancy you're not likely to see much but a sac.