This is the start of a potassium deficiency ... she's needing a good PK booster at this point - something like General Hydroponics KoolBloom liquid which has an NPK of 0-10-10... the dry formula of that is used in the last two weeks of flower and it has an NPK of 2-45-28... PK boosters really target the bud development and are usually started around week 3 of flower which is just where you are! Don't expect the damaged leaves to repair themselves, they won't - but if you get some PK going, the damage shouldn't spread more after a couple of days...
Good luck!