I cant tell from your pics, but did you lollipop your plant?
Like Jim said, the plant will begin to "self prune" the very lowest/unproductive leaves during flower, by depleting the mobile nutrients within them, causing them to yellow, brown, then eventually die off. This is actually fairly normal on plants that haven't been lollipopped (removing the bottom 1/3-1/2 of the foliage of the plant), and shouldn't be an immediate cause of concern if your only noticing it on the very oldest/lowest growth, and only in small batches.
You shouldn't, however, see the entire bottom half of the plant yellowing at once, as that would indicate an issue beyond the plant simply "self pruning". While she does use SOME nitrogen, even during flower, so long as your using your nutrients as recommended by the manufacturer, its highly unlikely that you'd run into a nitrogen deficiency... it would more likely be a salt buildup or something of that nature. Could even be that you splashed some nutrient water on that leaf, and it caused it to get a bit funky.
Just keep an eye on it, make sure new/upper growth is that nice, dark green color, and dont fret too much over a funky leaf here or there, especially on the oldest/lowest fan leaves. Keep up with you nute schedule, and watch to be sure the issue doesnt progress up the plant, but from what i can see, overall, your girl looks happy and healthy
Good luck to ya!
