so what you are seeing is your ph is off, you got it early so here is the fix.
next time you water, water until your run of is at a 6.0 ph reading, now let her dry out for a day or 2, when you water the next time use nutrients, a very small amount including cal mag in the mix.
your plant is at the age now as it depends on nutrients.
you should start seeing a improvement right after the first feed.
it is no different than your other grows.
hope this helps.