How soon to pregnancy tests determine pregnancy?

How soon is too soon? I took to pregnancy tests a week ago which would've put me at maybe 2-3 weeks pregnant and both were negative. But when I was pregnant with my son I took a test and it was positive 6 days after conception. I am late on my period and my have been sore for almost 3 weeks now. What other reason could there be? Is it possible it was too early for detection when I took the tests?