Please help...

I had an ultrasound yesterday at what should have been 7w 4d (I know the day of my LMP and the day I had sex and ovulated). The tech told me there was an empty sac measuring 5w. After speaking with the doctor, he ordered an HCG level, which came back today at 25,000, usually the level between 7-8w, which I should be. I go back tomorrow for a follow up HCG level (48hrs.) later. Does this make sense, and has anyone else experienced this? The doctor said he doesn't want to get my hopes up or give me false answers, so now I just wait...