yellowing leaves w/brown spots HELP!!!

hey guys my plant is 3.5 weeks into flowering and the leaves are starting to yellow. i know this happens towards the end of flowering but it seems a little to early. please look at the pics and tell me what you think. the temp is a constant 78 degrees

thanks

what nutrients are you using, how often, and whats the ph. it looks N dificient.

i'm using distilled water with liquid nute. dyna gro bloom 3-12-6. i use that then water with distilled water 2 times then the nute formula. i water about every 4 days. ph is stable i believe. i started flushing 5 days ago thinking that it coulda have a been a nute buildup from veg.

Man, about the 3rd week of FLOWERING on 12/12, sometimes the 4th week, those big leaves, the fan leaves, they start to die, turn yellow and fall off. As long as it is just the big fan leaves, don't worry about it.

its a lil more than just the big fan leaves. the leaves near the buds are still green. what is the proper way to flush a plant?