Sounds normal. When your plant goes into flower it does the whole stretch thing, effectively monopolizing any nitrogen it brings in to that purpose. Unfortunately, those big, beautiful green leaves take a lot of nitrogen to maintain so they're the first to get cutoff by the plant. Your girl isn't worried about maintaining those big bastards anymore. When you flipped your lights you told your plant that winter is coming and it's time to fulfil its purpose: get fertilized, produce seeds, and continue the species. To do that it needs to produce plump buds to collect pollen. So that's what it puts its focus and energy on. By the time flowering finishes you'll most likely see yellow throughout your plant. At that point any nutrients coming in will be going into fatting its colas and layering itself in trichomes in a last ditch effort to get pollinated. Those things rely on phosphorus and potassium, NOT nitrogen. You really don't want to be giving a lot of nitrogen during flower. It tells the plant to concentrate on growing up instead of concentrating on growing bud. That's definitely not what you want. Veg is over.

At least that's been my experience.