I would start by checking and keeping an eye on your ph. Should remain around 6.0 for this stage.
Second i would suspect an early mag def. Check your cal/mag level and balance. Ca should be twice mag.
Here is the guide I use to help me determine what I am seeing.
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In particular I would take a look at this one:
Does your cannabis plant have yellowing lower leaves between the veins? See pictures of a magnesium deficiency and learn the easy solution.
Brown spots appearing on your lower leaves? See what cannabis leaves look like as a result of pH fluctuations and learn the solution!