I can understand that dongles.
My friend's cat has just got his pelvis broken from being hit by a car.

I don't suppose there is a new cat in the neighbourhood is there? An uneutered tom bullying yours perhaps? A new dog or even a neighbour perhaps shooing her off that's upsetting her?

Could be that it is a comfortable spot in the heat.

I have never been without a cat for 45 years & am still surprised by their odd behavior at times!

Olly, my bravest one, would let any strange cats know that his garden was his territory, until one who seemed to be looking for a fight came round and attacked him. He became nervous and it took me about 6 months to get rid of his nerves!

Get a small water pistol and, if the stronger of yours starts a fight, give him a squirt! They hate getting wet so it will soon stop him fighting.
They are so territorial that they need to prove who is the boss. I am with them most of the time, so when one of mine does it I shout and say " Get to bed" and amazingly that is just where they go till they have calmed down.

Try spending a little time stroking the weaker one, to let it know it is safe where you are!
Good luck.

Yes I wondered if it was the heat too. I have three cats they seem to be coping with the heat ok but they are very fractious maybe it just too warm for him right now.

Our other cat has taken to sleeping in the bathroom which is cool on the tiles and to be honest, she hasn't been so mobile as she normally is either.

Maybe I'm just being over cautious.

They both seem absolutely fine health wise, they are alert, eating and drinking well.

When I got up this morning, she must have been upstairs. She came flying past me, straight up onto her usual spot, so maybe it is just a terratorial thing after all.

Maybe....Good luck at the vets anyway I agree it's worth mentioning.

Hope theres nothing wrong and its just cat being cat