I had late miscarriage at 22 weeks and 3 days

Patient: I had late miscarriage at 22 weeks and 3 days then delivered on 04.04.15. I have had a period since and detected 6 high fertility days followed by 2 peak fertility days. Me and my partner have been trying to conceive again and was testing negative from 8dpo. On 12dpo (yesterday) I got a positive pregnancy tests with 5 different tests. I went for blood tests doing and the doctor has called to say my HCG levels where only 27 so he would think this is from the previous pregnancy. Is it possible to get a negative test then a few days later get a positive test yet it be from the previous pregnancy? I understand only more blood tests will give us a definite answer but just wondered how this could happen

Symptoms: Late period, positive pregnancy test

Doctor: