Nitrogen deficiency is the most commonly occurring nutrient deficiency in cannabis.

Symptoms
Lower leaves first appear pale green. The leaves then yellow and die as the N travels to support new growth. Eventually the deficiency travels up the plant until only the new growth is green, leaving the lowest leaves to yellow and wither. Lower leaves die from the leaf tips inward.

...In the middle to the end of the flowering stage, plants frequently show a N deficiency. They're using the nutrients that were stored in the leaves and dropping their oldest, bottom fan leaves. To prevent the deficiency from getting extreme, switch over to bloom nutrients gradually unless the bloom fertilizer contains some N. - Ed Rosenthal

I am 4 weeks into flower, the shop for my strain states 7-9 weeks to harvest from flower. I do give nutrients containing N, so hopefully it's normal for me. That is Nitrogen deficiency though, and not toxicity.