Oh I have had exactly this! It really gets me down as in the moment it feels like all the training has evaporated and I question whether I have managed to teach him anything at all. But as you say, it's just one day.
@FoxandDuck mine needs to chew a lot right now with his teething. He will lay down by the baby gate sometimes and watch us (reproachfully) as we go about our business. And other times, especially mornings, he might just chill out on a blanket for a bit - maybe dozing, maybe just hanging out. Then other days he's manic or asleep in the crate with no in between at all. I think it could be overstimulation as Twiglets says because when I'm wfh and there is no one else around he is much more likely to settle and occupy himself, but if kids or visitors are around - or he's just having a mad day because it's windy/he's teething/he feels like being a knob - he can be far more frantic unless unconscious and those are the days I need to have a really good chew available to him that will help him settle. My conclusion on everything is that it's variable, and when we're in the thick of a challenging couple of days it feels like forever - but when I zoom out to bigger picture, things are better than they feel.
Any advice on lead biting, anyone? We have gone from beautiful loose-leash walking to gremlin hanging off the lead by his teeth and snarling. Why, I do not know, so back to fucking basics again!!
(I get myself through by promising myself I will never, EVER, do the puppy phase again - it's grown up dogs all the way for me from now on! Or maybe cats.)
I do love him btw.