Help please!

So I found out I was pregnant on the 24th October. This will be my 4th pregnancy (two children on earth and one miscarriage in June) I rang the early pregnancy unit because of some sharp pains and brown spotting I was having. Got saw and scanned the next day (Tuesday). Apparently baby is in the right place which is great! However they said they could only see a sack and it looked like I was only 4 weeks along?? Which is pretty impossible considering I was testing from 6 days post ovulation and got a positive for the first time on day nine.. I haven't had a period since the miscarriage, but got a positive ovulation test on the 14/15th October. I'm so confused and worried that the baby isn't growing right and I'm going to have another miscarriage. I've got a scan again on Tuesday morning to check on everything. So my question is, has anyone been through this?? Did it turn out okay in the end?

Thanks!