I punctured my arm on a bromeliad thorn. now several weeks later i have a hard lump under the skin like a large bb. it is slightly painful?

Foreign body: What probably happened is the tip of the thorn broke off inside your skin, leaving a foreign body (similar to a sliver). Sooner or later your skin will expel it, if it's not in too deep. If it is too deep, or you don't want to wait, a doctor (preferably a dermatologist) can remove it for you.

See a doctor: It is possible that you have one of several types of infection with inoculation of organisms into you by the thorn.