Yea,I got this experience fourtheen days ago,with Koxka my 9 yrs.old Half-spayed female.
My cats are freely feed;I own four;they are all healthy then I don't control the individual amount
of food they intake.Koxka has been always very
tiny,but strong and healthy,full of energy.
But when the summer is geting very hot,like these days,she always get a *crisis*.This year I noticed she wasn't showing up as ussually,she likes to be appart from the furboys,and retrived in the darknes coolest room of the house,that is a good thing to fight againts hot.But I noticed that she wasn't in a good shape and looking more skinny than usual.I pick up her sking with two fingers then see the skin retrieve as usual,nop her skin
hardly regain its place.You Know what it means,she was dehydrated!.We rushed to the Vet.
There she got anesthesia(she's well known at vet office,for being quite agressive),and did the all ritual:Testing blood,X-rays,but nothing seemed to be wrong with her.Only that she was obviously very dehidrated.The vet gave her a long shot of phisiological liquit?(suero fisiologico)with some antibiotics,left the catheter for me to give her
more of that stuff every evening plus a special diet of canned food called AD from Hill's Presciption Died,that is very smooth so you can force her to eat so with a syringe,without a needle of course!.And in two days she seemed to recover a little.Always with phisiological stuff and forced to it AD.Thanks good she liked that canned food,so when she recovered a little,started to eat by herself,her and me in the kitchen,closed door,no distractions!The others kitties came along attracted by the good smelling of the food!
Now he's already taken dry food but still needs to put on some weight.And a lot of rubs,cuddles,kisses and so on...
My advice is,make the skin test and go to the vet,
a dehidrated kitty can die in no time.

Koxka