empty gestational sac !?

Hi! I went to the hospital last Tuesday. They told me I (still) have an empty gestational sac. The sac was 15mm and we saw nothing inside. The doctor said it could go either way. I'm feeling nauseous, my breasts are sore... but that doesn't really mean anything. I also have no idea how far along I am. I had a negative test on July 7th and the first positive on July 10th. Anyway, I have to go back next Friday. Then I'll find out... Does anyone here have experience with this? I'm afraid the sac is really empty, the idea just feels so incredibly sad. Best wishes