Fetal pole appeared late - definitely miscarrying

What on Earth could be going on here? I miscarried in December and had a period on 14th January. I ovulated on 31st January according to OPKs and bbt. I got a positive pregnancy test on 10th February. I had a scan on Sunday when I should’ve been 8 weeks exactly from ovulation and I measured only 5 weeks 6 days with no fetal pole, only a yolk sac. Then, yesterday (Thursday) I measured 6 weeks and 4 days with a fetal pole and no heartbeat. I should’ve been 8 weeks and 4 days. We didn’t BD after 31st January so my dates can’t possibly be wrong. I know I’m miscarrying, though the EPU are making me go for another scan next Thursday to make sure. I just don’t understand why things appeared and developed so late. I thought this would be a blighted ovum.