it's a relief when that happens!! my tap water has 0.50ppm nitrites in it already so the nitrite peak was an exercise in frustration and water changes (fish-in). so happy to see that pale blue colour.
Congrats. You want to get several days of seeing it repeat that. You've been more than a month, so as long as it repeats, you should be cycled and not see any mini-spikes after the big water change. WD