i think adding a bit of nitrogen may help. I am torn between "might be normal" and "might be nitrogen deficiency". In my grow I woke up to "yellowing" of some bottom leaves, and then the next morning...it had turned 3 fan leaves yellow. Started doing some research on here and found it is a toss up of "some lower leaves do this toward latter part of flower and it's normal" to "nitrogen is still used during flower and plants might need a bit more nitrogen".
I chose the latter route because I had been feeding on the light side due to an overfeeding when I first made switch to 12/12. I use 3 part AN and had fed last 2 times at 1ml per liter. I have to feed this particular plant lighter because I have it in some pre-nuted soil I bought when I first began my grow and I didnt know any better. I increased my last feeding of this particular plant to 3ml per Liter and the yellowing hasn't advanced any further so far so I am starting to believe I was just lacking some nitrogen and it was taking it from my bottom leaves.
That might not be the case in what your plant is experiencing, but a little bump of nitrogen and monitor it closely for a few days to see if the problem continues or if it stops the yellowing and you get some noticeable results is pretty easy. But like member Lio pointed out in his reply, this could also be some sort of ph issue.