A single leaf turning yellow on one side only??

She is 16 days into flowering now, and i just spotted a single leaf that is turning yellow, but only on one side of the leaf? i've stopped feeding her and only give PH regulated water for ½-1 week, as i saw signs of overfeeding yesterday, some of my smaller leafs at the bottom are also turning yellow, but i expect it has something to do with the bigger leaves shading the smaller ones?

What can cause the yellowing on that big leaf? if it was lack of nitrogen it would have been on several leaves right?