Swayback, could you be thinking of Laportea, which is in the same family, also stings, but has some alternate leaves?
See:
http://www.illinoiswildflowers...
I can assure you that my pic is of stinging nettle (Urtica dioica) and it sure does sting.
I should add that I'm also thinking the original plant in question is likely neither stinging nettle nor catnip.
