Deficiency in flower overnight!

I fed as usual not too heavy, ph was a little lower than usual but was around 6.2/6.3, couple weeks into flower and this happens. Not too sure what to do, don't want to flush it as i gave it a good watering yesterday, but if anyone could point out the problem, i will find some way to fix it.

Can you show us under regular light?

How about more info...

What your medium

What's your ppm .

How close are you lights

Temp/humidity?

Did you check for bugs yet?

Seems to be a classic magnesium deficiency. Based on the timing of your grow, she sucks magnesium at this stage more than ever..it's generally the most common deficiency. What kind of water are you using and what feeds have you been giving?
By the time you get necrosis like that, it means the deficiency has been there for weeks. You may have corrected it by now and that's just the damage that was done prior. Do you have a pic of her say 2-3 weeks ago?

Definitely mag, that just popped up over night? It's pretty profound..

I'd say he noticed it overnight You don't get necrosis overnight..the yellowing was the first sign but maybe overlooked, now it's necrotic, but seems, from the 1 pic, not to have spread or gotten worse.