Whats the particulars of your grow ??? It looks like the beginnings of a Phosphorous (P) deficiency. Could also be a Magnesium (Mg) def. Freak
Kinda looks like you have too much nitrogen. When you have an abundance of nitrogen it pools up in the leaves and burns them, your leaves are also super green which is another sign. I could be wrong, it's best to test your soil. When in flower you don't need nitrogen, load up on the phosphorus. I use a bloom formula that is 6-30-30 for my flowering.