its a nitrogen defficiency and while a little yellowing very late in flowering is considered ok by many it is not natural for the plant to have yellowing leaves even at the end of flowering.
either your not feeding your plant anything, or your feeding it a bloom fertilizer that doesnt have sufficient nitrogen and many start it to early often when they switch to 12/12, its actually better to not start a bloom fertilizer until 2-3 weeks into flowering.
if your having yellowing problems early in flowering should probably read the link below.
http://www.drugs-forum.com/growfaq/1231.htm
bejay Reviewed by bejay on . Question about Flowering Stage I have had my plants in flowering stage for about 2 and half weeks now and they all have buds and alot of white pistles on them, but on most the big fan leaves they are begingin to yellow or already are yellow...is this natural due to it ending its life cycle or no?? Rating: 5