Too early or blighted ovum? Also blood clot in my uterus!?

I went for a 7 week ultrasound last week. I saw a sac in my uterus but it looked empty to me. The doctor said that he thinks this could be a unsuccesful pregnancy but he could be wrong. He did blood work and wants me back in 2 days for another scan. Could I have ovulared later than I thought and tgats why it looked empty? I thought I saw a heartbeat but he said that it was probably my pulse. Also he said there was a blood clot in my uterus but didnt give me any restrictions. That alone doeant sound right. Any advice to what might be going on??? I still have all my pregnancy symptoms and no bleeding. Thanks to all!