Still got a "period" but then also a positive HCG test?!

Hey guys I'm wondering if anyone else has experienced this and how it played out:

I checked on 11 and 12dpo to see if we conceived, but had no positive tests. I stopped testing because later on 12dpo i got what i thought was my period.

My period lasted 6 days and had no clots, but id still say it got to a "medium" flow. Then a week after my period ended i felt some cramping and took a pregnancy test and it was a blazing positive?! Dr. confirmed with bloodwork that i conceived from my previous cycle. I'm in shock.. what the heck was all that bleeding, anyone have experience with this?!