My best guess from your pics that show what looks like advanced Mg deficiency is that you already had the conditions set up for this problem and the feed just puched it over the edge.
If as you say you only fertilize minimally you should look at your soil as a potential culprit.
Flush with distilled water and measure the pH of the runoff.
Flushing was a good idea and it's good you already did that.
You can get Mg lockout pretty easily, and flushing is the first step towards fixing it.
Now you are going to want to make a foliar feed that contains 1/4tsp/gallon of epsom salts and mist your plant with that.
You have to do it right away before you get bud formation, because then you can't foliar feed properly after that.
For flowering you should be feeding with a lower N fertilizer than what you're using too. Also if you find that your soil pH was off, you need to be adjusting your water.