What the others have said is correct. You wont test positive on a HPT until your HCG level is at a level that can be detected by the test. Same with a BT at the doctors. You could still be pregnant but it won't show until just before your period is due or sometimes after it's due. The HCG level starts rising when implantation occurs and this ordinarily happens from about 6DPO onwards in most cases and then it takes a few more days to reach a level high enough to test positive.

I've had positive HPT's as early as 10 days past O, so around 3 days before my AF was due. I've never seen a HPT that says you can test as early as you have suggested and I've used a number of different brands, including ultra sensitive ones.