You maybe earlier than you think because if your to early you can't see anything
Have you been back to Dr? there was nothing? no sac or anything? I don't know if I have supportive words but no one has answered.... It could be a few things really it could be ectopic if you saw sac at all it could be a brighten ovum where the baby doesn't form or you may be earlier then you think witch it wouldn't show anything yet on the u/s. Good luck.