Hi there and thanks for your question. So sorry to hear that your cat is having these problems.
It is very unusual to see something like this after a spay, but I have a few possible ideas for causes.
I am wondering if it is possible that your cat had a period of anoxia during the anesthetic. This could happen if the anesthesia technician had the oxygen level too low. It is conceivable that this could happen and there may not have been any obvious changes on the monitors during the surgery. Or, it may have happened and the technician may have covered up the mistake. If there was a period of anoxia then this can cause damage to part of the brain.
The good news is that if this has happened, there is a good chance that her brain will recover and she should be fine, although I can't say for sure if this will happen.
The next possibility is that there was some sort of neurological problem present and the stress of the surgery brought this problem to light. One such possibility is a disease called FIP. FIP can cause little granulomae (similar to tumors) to grow in different organs including the brain. It is often difficult to diagnose. There is a blood test that can be done. If the test is negative then it is NOT FIP, but if it is positive it may or may not be FIP. I am hoping this is not FIP because we don't have a cure for it.
One final possibility is that your cat has a heart condition such as something called hypertrophic cardiomyopathy. This is a condition in which the heart walls are thickened. It can often show no symptoms. It is possible that your cat had an "embolic episode" under anesthetic which is similar to a stroke in which a small blood clot affected the brain.
Unfortunately, it is going to be really hard to say what the cause is and whether she will fully recover. You may want to ask your vet for a referral to a neurologist if there is one in your area. Or, if you are suspicious that your vet may have been negligent you may want to ask a second vet in your area to examine your girl and review her records.
I hope she does ok!
I hope this information helps! If you have more questions regarding this problem, just hit reply. Or, if my answer is sufficient, then please hit accept. Thanks!