Plant turning yellow 3 weeks into flowering?

I just started flowering Feb 1, and until the second week into flowering everything was going good. Then, this last week and a half it's been down hill.
I think I had a toxicity problem at first, so I flushed to be sure. But flushing I think just made the nitrogen problem worse. I also think it might be a pH problem since I was using only distilled water.
I recently got a 5-15-14 maxibloom powdered solution to counteract the pH problem and add nutrients. The maxibloom makes distilled water 7.0 from totally alkaline, and if I add it to well water it makes the pH 5.5. Should I use well water or distilled?

Also using a 1000w hps that's about 2 feet away from the plant. the plant is in a 5gal bucket. The yellowing first started in the middle of the plant on the fan leaves.

Will my leaves return to their green color or is my plant kinda F'd? Pretty sad sight to see since it's such a beautiful beast, really would like to see a rebound.

Where about did the yellow start from? Also, how close is your light to your tops?

Mid plant first affected, I have a 1000w hps about 2 feet away from the tops

You might get people telling you it's nitrogen deficiency, this might be true but nitrogen deficiency starts at the bottom of the plant. I've recently had Lime green/yellowing problems at the tops and it turned out to be light bleaching, if this is your problem just move your light up a few inches each day until your new growth goes back to normal colour. Though on a positive note your girls are looking great!

I also thought my HPS might have been too close because my 3 main toppers to the plant seem to be the most affected. However though, I have leaves that are in the most shaded regions at the bottom of the plant that are affected by this yellowing problem. These leaves in the shade obviously aren't being bleached...right?
These lower leaves didn't yellow at first though like you said about typical nitrogen deficiencies

Those shaded leaves could be of the same problem just they grew under the new growth. Though it's not like your plants showing any serious issues, plants tend to yellow in flower as they use nitrogen stored in the leaves as nutrients, so if you've tried moving your light and nothings changed there's no harm in giving your girl a half dose of nutrients in her next feed to see what happens.

Keep us posted on this, I too am dealing with the same issue and it didn't present itself until the switch to hps. Working with an air cooled 600 watt so when the switch was made I kept it as close as the mh (6") and within a few days I noticed this bright yellow starting at the center of leaves and some bud sites. The growth rate seems to be unbothered at this point (Day 15 since switch) but the coloration has me worried. I have raised the hood to 12" but I don't see much change yet and new growth still seems to be yellow in the affected areas. I'm afraid things may stretch too much if I raise it too far. This is my first rodeo so I have no past experience to compare to but up until this point I've had the bull by the horns.

I am going through the exact same problem also. I'm 23 days into flower (or about 16 since the plant actually began budding) and just in the last few days some of the fan leaves are turning yellow. I talked to a friend who's been growing for 10 years and he said that in the first 1-2 weeks of switching to 12/12 that you should still use veg nutes (which have a high N content). He said it's an easy fix and to go ahead and give the plants half grow nutes and half bloom nutes with your next feeding.

Flush it for 2 weeks, cut the ppm 25% on the next feeding. See what you get

How shows nutes def table yellowing middle leaves means Molybdenum deficiency.

The ph being wrong is the reason and now its too late. In future make sure all feeds and waterings are 6.5 right from the start of veg.

If you are running a soiless mix, I would go to 5.9-6.2 PH.

Did.u. Transition her with half veg half bloom