during flowering all the plants energy goes into producing bud. The fan leaves at the bottom will die for two reasons the plant is putting its energy into the buds and also the fan leaves will not recieve as much light as the rest of the plant will cover them. Have u got any pics cos if u say its going up the plant it could be rot but i doubt it as u say the leaves are drying up. Let me no if u have any pics. I wouldn't worry to much, harvest when u want but as long as thr buds look healthy then leave until right time to harvest. All the best.