Hemp DOES contain THC and todays drug tests can register even the smallest amounts.
It would be hard to prove that you did NOT smoke a joint two weeks ago, but instead used a hemp product 1 day ago.
Maybe you used one to hide the use of the other?
Hemp = cannabis, the "normal" hemp plants usually contain less then 2-3 % THC.
Thanks to the prohibition the psychoactive strains have crossed the 20% barrier in the late 80s, early 90s.