Miscarried on 8-13

Misty

I officially miscarried on 8-13 after starting to bleed on 8-12. I went into confirm what I already knew to be true on 8-16. Doctor confirmed I had a completed miscarriage, but my hcg level was still 328. I saw watery cervical mucous on 8-28 and 8-29. I had unprotected sex on 8-30. I have no sign of AF and I keep getting a BFN on tests. Do you think the neg is accurate or do you think it might be to soon to tell?