OK, I'll go over some of the top possibilities and you need to rely on your instincts to decide your next move ok?
When a cat stumbles, takes a few steps and falls over or generally seems to be off balance, a number of causes may be involved. It would take an experienced veterinarian to really determine what's going on based on physical examination and blood testing for various things such as infection, virus and so on.
When we know that a cat has a questionable background with nutrition, such as a previously feral cat (many of our rescues are born to abandoned cats and have never had good nutrition) we see this symptom as the result of deficiencies.
In other cats this can be the result of an inner ear infection or even a tumor effecting the brain stem, cerebellar regions or possibly feline ischemic neuropathy. This is sometimes a symptom related to Feline Leukemia Virus, Feline Infectious Peritonitis, etc. which are diseases that may cause central nervous system symptoms such as this.
Feline Vestibular Disorder is generally accompanied by rapid/or otherwise abnormal eye movements and is usually pretty sudden onset. With or without treatment it tends to self-resolve in less than a month, but it's my opinion that a sure diagnosis needs to be made first. Otherwise, if this is something more serious (tumors, viral infections/bacterial sources, even liver problems) it may become more complicated to treat, if treatable at all, by delaying it.
Modifying your cat's diet to better follow feline nutritional needs is important and how to do this may be surprising to you. Unfortunately, many vets don't have the background they should have when it comes to advising on feline nutrition (even if they're selling a designer brand food).
A vet who is specialized in feline nutritional needs has become determined to provide everyone the correct information without ulterior motives (no sales), including recipes for homemade options and comparisons/evaluations of cat foods - take a look here http://www.catinfo.org/
It must also be considered that the cat is suffering low blood sugar/hypoglycaemia, a blood clot or may be epileptic (though less likely).
If eyes are fixed and not responding, if there is labored or difficulty breathing, pale gums, lethargy, drooling, vomiting, diarrhea (especially with any signs of blood), or any other symptom that causes you concern - it is far more worth taking the cat in right away than to hesitate and possibly lose your companion.
It sounds like there isn't much possibility she's been exposed to a toxin, but sometimes this is what's behind it. Even ingesting certain plants might effect one cat and not another.
Symptoms of B1/thiamine deficiency in cats or kittens are often lack of balance, weak appearing back leg function and what is described as "walking sideways and falling over".
Feel free to share this with your vet:
Measuring erythrocyte transketolase activation after the addition of thiamin pyrophosphate is the standard (biochemical) test for thiamin deficiency. Some clinicians who see a large number of feral cats will know upon presentation that the cat is nutritionally deficient.
Neuro-anatomical changes when a feline suffers a B-1 deficient diet might include seizure activity, ataxia, gait disturbances and even mental retardation. Some kittens born to a deficient mother will show symptoms from the time they can walk. Often, injections or supplements of B-complex, B-1/thiamin will lead to almost immediate results (within 24 hours in many cases).
Cats who are frequently fed fish or seafood might suffer thiamine deficiency, or cats eating dog food (could she be stealing some of the dog's dinner?).
The vet should do a CBC, biochemical profile, possibly urinalysis and sometimes X-rays are done to rule out injury or defect.
Just in case you need some help, the list below should get you started. If you don't need this list, keep it in mind for your friends or anyone who might. I want to thank a very caring expert (Terri) for sharing
American Animal Hospital Association
http://www.aahahelpingpets.org/home For veterinary care for sick or injured pets if they have been abandoned or if their owner is experiencing financial hardship
Angels 4 Animals
Services range from financial aid to complete treatment
to those pets and pet owners in need.
A credit card company for health care, including veterinary care.
God's Creatures Ministry
They help pay veterinarian bills for those who need help
For the elderly, the disabled, the working poor.
Feline Veterinary Emergency Assistance (FVEAP)
For seniors, people with disabilities, people who have lost their job, people who rescue a cat or kitten.
Also, don't forget to call your local Humane Society to find out what they might have in place, including clinics or discounts at certain vets.
Call your local Pet Smart and Pet Co to find out the names of rescue groups often working through them - they will have names of vets or other options you may be able to use.