BFP super fast and dark.

I took a hpt when I was 3 days late. It came up positive extremely fast (within seconds) and the test line was even darker than the control line. I understand that HCG levels get stronger as pregnancy goes on but i had taken a test 10 days prior (just felt “off” and had a “gut feeling”) but it was negative. So I don’t think I’m further than I think? What else could cause HCG levels to be so high so early?