Same question as Purple, what happened to the fan leaves?
Drawing from my vast experience (in the middle of first grow), that's a way out of control cry for nitrogen. Stoney's right about some yellowing being normal, but it should be a gradual process starting with the lowest fan leaves, working its way slowly up the plant as flowering progresses. But that should be limited mainly to fan leaves, and it looks like in your case just about every bit of nitrogen has been sucked out of every one of your leaves. If it were me and it still had a ways to go, I'd give it nitrogen and hope for the best. But if it's almost ready, I wouldn't wait to pull the trigger because if it's as sick as it looks, it's not going to get any better, and it's going to be susceptible to rot or anything else that comes along.
Good luck and keep us posted.