A mystery to me (photos) any insight?

Hey all. I have an apple mush pictured here. I suspect it is nutrient burn however, I haven't fertilized for 10 days and have only used good water since then. I suspected burn about 9 days ago. I didn't do a flush, per se, but I have watered three times since noticing the burn rather heavily to encourage run off. Yesterday I watered again and this plant got very much worse since then. Could the water be releasing pent up nutrients? I'm confused. There were no signs of that burning/orange tissue on the small leaves next to the buds yesterday. I'm worried other plants may start reacting like this. Any thoughts?
Also. I did a round of mighty wash for a very minuscule mite problem. I sprayed right after lights out and washed the residue right at lights on the very next cycle.
My specs are listed below. Thanks in advance.
Ps. This plant has always been somewhat of a runt.

2 600w hps
fox farm ocean plus ff nutrient at about 1/4 recommended online chart
Apprx. Week 5 of flowering....light went 12/12 December 23- today is January 29
on temp 80
off temp 65
humidity 35%

Am I to believe that no greenthumb gurus have any input?

Not overly familiar with the nutes etc you are using, but given the fact that you have stopped feeding for 10 days, and new growth is rust free, and existing rust patches arent getting any worse, and pics dont look that bad really---could be that rather than looking to fix an existing problem---you have already fixed it and the plant is in recovery

Thanks for the input, Bilbo Baggins. However, she did get worse just after a recent watering. I'm wondering if the watering is maybe releasing pent up salts/ferts? I suppose I should just flush them all. I'm also concerned it may be a rust/fungus. Any thoughts?

check out that ffof uses powder oyster shell as a buffer
as the plant is watered, the oyster shell can runoff. From here, you see a pH drop and then it locks out cal-mag

the fix to this, is if early enough, giving it a transplant (which it doesn't look like it needs) or pre-mixing a little dolomite in the first place
you could also take fine ground dolomite, try to dissolve in hot water (it's not supposed to dissolve) shake the shit out of it, make sure its ~68 deg then water it in

to correct by flush, you'll need a meter, either ph or ec (ppm)
run water through , slowly, until the ph or ppm comes out the same as it is put in. After this, apply 1/2 dose ferts (which is now going to be much higher than before, because there are no more nutes in the soil) then step up to your full strength.
I never follow the bottles exactly, just use em as a guideline. I do 1/2 or so of recommended maintenance amount, then work from there. PH is now important for the water used because nothing is in the soil buffering the pH anymore

i found a mixture of happy frog and ffof does well, if i was to use ffof , i'd transplant right before flowering, to make sure it has the most time it can have in that new, buffered soil

PH, my friend. Flush and adjust. That's my opinion.

Did the spots start at the top or bottom of plant?

Thanks for the replies guys. Last night I flushed that one and another (same strain) that is exhibiting the same issue, albeit milder. My soil ph was a bit low, 6.0. I feel like giving it a day or two to see any results before adding any more fertilizer, as I'm slightly apprehensive about its effects. I can only assume that either the Mighty Wash had something to do with affecting the ph (or perhaps certain patches were missed during the rinse and the plant scalded) or that the Tiger Bloom I added burned them. FF recommends 2 tsp per gallon and I use .5tsp per gallon, so 1/4 strength. Hopefully that wouldn't burn?

@Nizza: Thanks for the insight on The FF soil. As soon as these are done I am going to use Ocean mixed with Happy Frog and add some dolomite. I read it is necessary to wet the dolomite and let it sit mixed with the soil for a few days before transplanting. Is this something you do? Also, I transplanted into the FF soil one week before 12/12 so they have only been in it for 6 weeks. I hope that isn't enough time to leach out buffers!

@Jimdamick: I checked ph and it was a bit low but nothing too drastic. It's a matter of finding the culprit now.

@Dr Gruber: The damage started out at on the older fan leaves, which led me to first think it might have been nutrient burn. A couple of days later I noticed those rusty spots and upward curling/burnt leaves near the bud sites. Any thoughts?

I agree with Nizza for the most part.
I used to use FFOF and had problems start between week 3-5 of flower. What i found is that i had to flush them really good at the start of flower and then continue with veg nutes for the first 2-3 weeks of flower before switching to the bloom nutes.
The reason I asked about where it started is that major and secondary nutrient deficiencies start at the bottom and work up the plant. Micro nutrient deficiencies start at the top and go down. Since you hadn't given any nutes for ten days i thought it might be cal or mag deficiency and could be corrected with epsom salts or calmag.
Either way i would still flush the plant and start over with nutes. After the flush a good balanced nute like a 20-20-20 is great for the plant. then switch to the bloom nute.
As Nizza said you should buy a PPM meter and a PH meter. They will teach you where you're at as far as nute levels per teaspoon and what your tap water PH level is. Also helps to see salt build up when you measure run off.

I always transplant about 14 days before flower.

Of course it certainly could be something like light burn from the wash but i would put that as less likely. I would also think it being a Ph problem is less likely as well.

P.S.- After a flush-immediatly add the nutes.