I'm going to go out on a limb here and say that it MAY -this is kind of a stretch- actually be Nitrogen def. despite the heavy N feeding... If you SUDDENLY gave your plants a TON more light than normal, and they got a sudden growth spurt, it stands to reason that they could sorta outgrow their N supply.
Try foliar feeding with a high-n fert and see if that helps.
Oh and of course check your pH...
I have NEVER seen that come on so suddenly!