your plants aren't fully in flower yet and still phasing over from a veg state to the flower state because of that they need less nitrogen than veg but more than flower
you should be able to add a little of your veg feed in the next to up the nitrogen levels a little
it's nothing to worry about and will recover easy enough over the next few feeds
also just because you have a slight deficiency don't put loads of nutes in next time, it's easier to add more than take some away