Ak 47

It's hard to say what the issue is with out seeing the leaves in question. It could be simply transporting nutrients from lower leaves that are receiving little amounts of light. This generally happens as the plant grows and those leaves lack light. If i had to guess with the lack of Nutrients and that your in flower and its starting to show now, it's likely a deficiency of potassium. Although calcium and magnesium deficiencies could look like burn Aswell.