Overnight u explained lethargy

We have a lively 6-mo old boxer puppy. He loves to play with his squeaky toys and play tug and run zoomies and overall just have fun! He completed 8 weeks of puppy training last week and does well for his age aside from seeing every dog as a potential playmate. He spends time outdoors but mostly indoors in the AC when it’s hot. All of a sudden, overnight, he became lethargic. Listless. Not interested in his toys. He will eat and his pee and poop are normal. We brought him to our vet. He was a little livelier with her and her staff. He checked out fine. Bloodwork normal. He came home and spent the rest of the day sleeping.
We are fearful something is terribly wrong. Any thoughts? He is up to date on all vaccines. We watch him like a hawk and don’t believe he’s gotten into anything dangerous,
Many thanks!