Lethargic 9 month old, growth spurt?

I've got a lethargic male (fixed with all shots up to date) that is otherwise heathy.

He eats, drinks a normal amount of water and has normal liter activities.

He just suddenly went from crazy energetic to sleeping all the time but no other symptoms. I've felt his body and belly and no pain or tenderness.

He has gained about a pound over the past few weeks (from five to a little over six, he was five pounds when I got him at six months). Is it possible he is tried from a sudden growth spurt? I'm not sure if cats even have those...

If this is very recent and hasn't lasted very long I wouldn't be too concerned.  If it continues then I would take him in for a check.  Kitties are very good at hiding that they don't feel well and a change in behavior is sometimes the only thing that we see.

He's been like this over a week now with no signs of improvement [emoji]128546[/emoji] today he had started walking oddly, like he's stiff or has a stick up his bum. I've made an appointment with the vet later this week and just hope it is minor.

Does he go outdoors? Good you have vet appointment scheduled. Let us know what they say.

No he's indoor only, I've also noticed his third eyelid is showing too in both eyes

Call the vet and let them know of this new issue and maybe you can take him in sooner.

I agree, he should be seen as soon as possible.

There aren't many things that will cause this, and a week of lethargy is something to really be cautious about, he may have an infection going on that can only get worse without antibiotics, I would ask for a blood test and urine sample to see what might be causing this.