What's your medium (and brand).
What's your feeding schedule lately and nutes.
Did you possibly splash anything on the leaves and is the leaf in your picture from the bottom of the plant, more towards the top?
My guess is you stopped "heavy feeding" and switched to the 2 waters/1 med feed when you started to see the discoloration on your plant leaves.
This combined with the soil age (been in same dirt since veg?), my guess is the soil is ph balanced but was or still is hot. Meaning too many nutes in the soil causing the plant to "burn"
I'd stop all nutes at this point and focus on just straight water, perhaps even a flush.
Just my guess though since the details involving specifics about your nutes (brand) and your definition of "heavy feeding" is fairly ambiguous.