they want just a little nitrogen. that's 101a nitrogen deficency. the plants budding, it still needs a little nitrogen. you've only been feeding her bloom. she is using up her own nitro. classic example, starting at the bottom with the lower fan leaves, during the heavy onset of flowering. give her a very tiny dose, then wait THREE days. you will see the leaves turn green again. BUT to much and your buds will stretch. so just a little then wait. mine take 3 days to turn back to green.
This guy is so dead balls on it's scary. It's normal for the bottom leaves to start to turn a little yellow in flower, but it's like fdd2blk said, N deficient. Ppl tend to get a little crazy with the Phosphorus when flowering and leave out the N and the K which the plant still needs, just in lower doses. This is something I copied a ways back. Don't remember who to give the credit to.
"A common mistake for growers when they reach the flowering stage is to start hitting the plants with a high P fert like 10-60-10, continuing to us this blend exclusively, and when their plants start experiencing a deficit of N or micros as reflected by the dropping of lower leaves and chlorosis, they wonder why. Plants flower as a response to long nights, not because of fert blends high in P. A ratio of 10-60-10 is wat too high in P. The plant will only take what it needs and compete for the other elements that may be MORE IMPORTANT at the time. You may have heard that too much N can inhibit flowering. No question about it, exclusive use of a plant food that is rich in N such as Blood Meal, a 5-1-1 blend, or ammonium nitrate may inhibit flowering especially if the Phosphorus level is low, but most balanced blends have sufficient amounts of P to do the job. Using a high P fert exclusively during flowering can actually work against you. It's an abundant amount of healthy leaves going into 12/12 that produces a lot of buds, not high P ferts."