Bottom leaves dying! Please help!

I'm two and a half weeks into flowering and my bottom fan leaves are yellowing and dying. The fan leaves on the bottom are however much smaller than the upper ones. During veg. its only light source was the sun during daytime and just one cfl at night to keep her from flowering. Now ive got her under a 300 watt hps. I m guessing that its no big deal and since it's flowering now the plant is just using up all the nutrients stored in those leaves but i want to be sure there isn't something else causing it. Any imput or advice would be appreciated.