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?