No yolk sack on ultrasound

My HCG levels were low but still in normal range. I have had the fatigue, nausea, swollen ankles the works. I went for an ultrasound at 7 weeks 3 days due to low HCG (17000) and the sack was empty. They said I could be earlier then thought so to redo ultrasound in a week. I had a headache for four days and was very ill off and on so I went to emerge. They redid the ultrasound again 7 weeks 6 days and again nothing. I’m supposed to be 8 weeks 1 day today :( my HCG levels are more than 23000 but they are calling this a blighted ovum. Where the sperm and egg meet and the sack forms but nothing goes further. I was looking it up online and it seems odd to me that my HCG levels are as high as they are because it sounds like having a blighted ovum would be earlier? Has anyone ever had this? This will be my second miscarriage. Hubby and I are devastated. I have had no cramping and no bleeding.