BFN but still pregnant?

Around 8yrs ago I was 10 days late for AF. I was getting BFN's whenever I tested. AF did finally start but was very heavy. A few days previous I'd had some blood tests done, for something unrelated to pregnancy. The day I went for my results, the Dr looked confused and asked if I was pregnant. I said no as I'd just got AF, but that I had been late. She said the blood test showed raised HCG levels. She determined I must have had an early miscarriage. I was so confused as all of my pregnancy tests were negative & I had zero symptoms.

Today, I am 9 days late for AF. Lots of hpt's taken, all BFN & no symptoms. Is it possible to still be pregnant with BFN's on hpt's?? And why did my blood test all those years ago show raised HCG when it never showed on hpt's?

Has anyone else had any experience of this? I'm going to ring the Dr and ask for a blood test to check my levels. I know for a fact I ovulated on the 29th Sept, so surely my body should have got rid of the egg by now.

So confused! Any advice would be greatly received x