It can be many things.

But if you started in January, that makes your plant about ready to harvested soon, yeah? (I should hope so, five months is a long time), it could just be the natural finishing process. The plant puts pretty much all of it's energy into finishing flowering, and will take away from leaves and whatnot.

But if it's NOT near finishing it's life circle (I don't know of any cannabis plants that take longer than 5 months, yeesh), then it could be any kind of disease/bug/nute burn problem or something.

I don't know all the facts, I'm still studying botany. Haven't graduated yet. :P

But that's all I can guess. Other people here should know something.