Likely it's ph swings from trying to correct a (runoff) number you're not sure about, and possible overwatering, and I'd back-off of the Big Bloom to about half of what their schedule says. (you are using the FF feeding schedule, right?) Keep your ingoing ph steady, perhaps closer to the 6.8 to 7.0 mark. But this is exactly where a ph pen is so handy. (to check runoff, which tells you a lot of what's going on in the soil) Commercial potting mixes lose their buffering capability right about, or just before harvest time.

If using their feeding schedule, did you add the Grow Big a couple of weeks into flower? Not really a big deal but if you want to, you can give one a half a dose of Grow Big next feeding day, along with her usual regime, and see her reaction. Tiz a good learning experience and isn't anything that would risk harm. Might stretch a tad, but have the ability to fill-in. Getting too late to really worry about the yellowing though. It's normal. The spots look like where moisture sat and burned. I do it all the time while being careless with splashing or dripping nutes.

The more you squeeze the roots, the more difficult it becomes for them to uptake the necessities, and the less room there is for nutrients to absorb into the medium. (less medium = less absorption...more roots = less medium) Restricted, sure...rootbound, no.