I am sorry your girl is not feeling well. I think the dry food may be hurting her mouth. She could have an abscessed tooth or root in the gum. When cats have pain they get upset because they do not know who or what to blame.
In a cat her age, my first suspicion in abscessed teeth in the gum giving her pain. The next is acid reflux. Then there is hyperthyroid, dehydration, renal and liver disorders.
There are a few possibilities for these symptoms. I will try to cover everything.
Weight and hair loss is usually related to hyperthyroid. The cats eats great but the excess thyroid hormone burns up all the calories.
The problem is that this disorder can lead to your baby developing diabetes, high blood pressure and other ailments.
We do have excellent meds for this but is is necessary to do a T4 thyroid blood test.Any other thyroid test can be inaccurate.
It can also be related to renal issues especially if there is excess shedding because the hair becomes very dry.
Stress weakens the immune system and can lead to illness.Dental disease also depresses immunity and sends bacteria into the organs.
You are absolutely correct to be concerned, dehydration is very dangerous.Sometimes it is caused by renal issues that begin with UTI's
Not only can infections damage your baby's urinary tract but her immune system as well.
The type of food is VERY important. Hi protein and MEAT puts weight on without unnecessary fillers.But protein is contrainicated if the cat has renal disease.
In addition, UTI's must be treated with an effective broad spectrum antibiotic like Clavamox or clidimycin and for a long enough duration so all bacteria is eradicated. If not, they will refluorish, stronger and more resistant and reinfect her . In addition, the bacteria can become tolerant of the same antibiotic so the medication should be varied for best results.
It is better if she does not eat dry food. Canned will ensure she gets adequate fluid. Cats her age can become dehydrated more easily. A dehydrated cat is not motivated to eat.
Cats that eat dry can have bouts of dehyration.Indigestion is caused by inadequate chewing. Vomiting is very common. When acid reflux builds up in the stomach, they begin to vomit. Even if they have always eaten this,, they often become intolerant to corn and gluten which are used as filers in most supermarket cat foods.Vets push science diet because they sell it but it is not a great food as it too contains corn and gluten.
Lack of fluid in the bowel also leads to vomiting, Ingested hair can also be a factor.Anorexia for any extended time leads to liver damage and causes nausea and vomiting as well.
I would add some canned or baby food for her
Serve the two foods separately - do not mix them - add a half pepsid a/c tablet, crushed up into the wet food one time a day.
Put hairball remedy or even plain vaseline on her paw for her to lick off as well.
If her tummy needs a rest, feed baby food with no onion or garlic for a few days.
If she wont eat, and your cat is not diabetic begin by giving her two droppers (cc size) of unflavored pedialyte every ten minutes for an hour. Next water down(plain water) canned cat food or baby food chicken or turkey with no onion or garlic and give only one dropper every ten minutes for another hour. Wait two hours and repeat both. When she feels hydrated she will lick up the gruel on her own.
She should also be checked for:
All of these are treatable so try to have her seen as soon as you can.
I hope all will be well so please let me know how she is doing.
Sincerest best wishes,