Could be a number of things......need more info & maybe a pic....my leaves are turning yellow & the plant is only going on week 3 of flower, no where near done...in the process of correcting that problem now
It is the natural process later in flowering, the plant senses the end is near. It is giving it's stored sugars to the new flowers to complete it's reproductive cycle. If it's still early in flowering, you should feed it some (N). But in short, it is a sign harvest is near. Post a pic!