When a plant yellows from the bottom up, 99% of the time it's an N deficiency. It's actually very common. Even though you're in flowering the plant still needs/uses nitrogen. What's happening is the plant is cannibalizing it's self. It's feeding off it's stored up nutrients.
Myself, if I had a plant in flowering showing N deficiency and I was using chem fertilizers, I'd give her a 1/2 strength dose of Veg nutes. Be careful though - if your veg fert is real high in N, use something lower. It's a fine line though. Usually with mj less is better, but too little and the plant continues to starve. Too much burns and slows growth to almost a dead stop.