Could be adjusting to the different lighting, but I'd think the tops would show the stress too, were it only the light.

Doubtful it's ph, as you are using fresh soil, right? But I'm guessing you can find your local water ph online, to see if your tapwater is within cannabis range. Tiz an important number to know.
Did you fertilize it before just before or just after the transplant?
I would let the leaves do what they want to do, and remove the clutter as it falls off. (bugs multiply under the leaves that are sitting on the soil)
Are you letting the soil dry between waterings?
What size pot is the 18" plant in now?