Yellowing/drying leaves after stress on early flowering stage.

overdrying in soil just creates salt build up, you overfed plant, now its aftermath. stop feeding and water with run off, plain water diet for few we/dry cycles at least. be wise, dont do any flushes in soil, it brings more problems than sort things, its for coco or hydro. lift pots, dont do basic mistakes as under or overwatering, have good regime . plant will look sad and harvest will be influenced by those mistakes.

This looks a lot like the plant is taking the mobile nutrient Nitrogen in the lower leaves and moving it up into the new growth. It may just require more feed and better watering, the older yellow growth is not likely to bounce back. But should stop progressing once you address the issue. Good Luck!

FYI my NPK now 6-13-18

I had similar issue though I grow in coco and my plant was over fed with Nitrogen from the very start (and it still has dark green leaves). Leaves were yellowing without any marks of deficiency, and falling off just like leaves on threes in a fall. The run off pH was way to high, I was pulling my hair I didn't know what to do until I was told that my feed is too weak. I increased feed (and also switched to another nutrients brand) - and in 4 days everything was back to normal. I am still trying to adjust to perfect EC, but no yellow leaves any more and I have new pistils growing.

This is normal for an over dry back to shed leaves. There is too much damage done to leaves sometimes and the plant knows it, so it will drop them. However, if this is progressing more and more, it is looking like the signs of a nitrogen deficiency as well. If you are seeing otherwise healthy leaves start to turn yellow near the bottom, the plant needs nitrogen. I hope this helps.