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Has she shown any other problems recently- circling, confusion, changes in vocalization, changes in appetite, weight loss?
While there may be a tumor, I doubt it has been there as long as the breathing problem- they most likely are unrelated. Diagnosing a brain tumor is difficult in that it takes specialized imaging tools to see it, usually a CT scan or MRI. From a practical basis it probably doesn't matter since brain surgery or gamma knife surgery, as done in people, isn't available. Brain will cause progressive disease with more frequent seizures and, in time other neurological signs that will be constantly present. Examples would include circling, blindness, head pressing against walls and general confusion. There are many other things that can result in seizures, so I expect your vet may do some bloodwork tomorrow to check. If it is indeed a tumor the prognosis is very poor.
Treatment of seizures in cats is usually phenobarb first, unless there is liver disease. A second choice, though more expensive is Keppra. These products are usually not started unless your cat is having an average of one or more seizures per month, clusters of several in a few days, or seizures that last for 15 minutes or more. In general it is better not to intervene during a seizure since you put yourself at risk of injury for no benefit to the cat.
Here is more reading for you: http://www.sniksnak.com/cathealth/seizures.html
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