Nothing to panick about just yet.
Im not the most reliable person when it comes to diagnosis but I'm thinking you have the start of a magnesium deficiency.
Going on what I've seen in the past too much megarop can do this.
Your plant looks pretty dark green which is also an indication of possible over feed.
How many grams/gallon are you feeding?
It could just be a simple case of feeding her a little less.
And she's definitely grown where you topped her.
That stump where you cut won't grow now.
You're looking at the 2 stems either side of where you cut, they are your new tops now.