Could be multiple things... PH is off, nute imbalance, watering cycle is off... I think (no guarantees tho) because she is transitioning too flowering, there might be a N shortage, why? Because of the stretch, wich is basicly intensive growt, she is making up for the shortage by pulling what's needed out off the other(older) leaves...
To avoid this in the future, keep giving grow nutes during the first 2 weeks of flower...