Negative tests 12 days late

This has never happened. I've been at the LATEST 6 days late but now I'm going on 12 days late. I have taken two tests, a cheap one(negative) and then this morning when I first woke up I took the clear blue digital (not pregnant) so I'm just trying to figure out if I'm definitely not pregnant? Or if maybe my hcg may not be high enough. How late were you when you finally got a positive?