I know the feeling of being "about to have a seizure." I even listed it as a symptom to my doctor (it actually made him grin because of how absurd it was!) There have been times when I was just so sure it was going to happen though. But it never has. As others have said, most people don't feel like they're about to have a seizure. And even if they do, it comes on very soon thereafter. So if you have a feeling, and nothing happens, that's a good indication that nothing is going to happen. But especially if you've seen the doc and he's not worried, why be worried yourself?
Try to think of this way - compare the number of people who get a brain tumor each year of ANY kind (including kinds that have spread from other cancers) to the population. What's the likelihood of a person getting one? Very very small (like, less than .001%). Okay, but now let's throw in all the factors that one has to consider, like having seen a doctor who is not concerned, having only vague symptoms - nothing catastrophic, etc. Once you add in all of those factors, that small percentage will have gone down that much further. It gets to a point where worrying about it is just ridiculous.
When you get these feelings, just try to calm down and remember the power of anxiety. I know it's hard, because the feelings we all get are so very real. And our minds immediately jump to the worst conclusions about the smallest of things. I don't know how long you've dealt with this now, but for me I've been facing anxiety issues for a year. The first symptom I ever had, I suddenly felt like I was going to pass out. Like, I really just had the surge come along and it felt exactly like that's what would happen. Since then I've had a host of symptoms. That "about to pass out" feeling, brain zaps, feeling like I was going to have a seizure, feel like parts of my body were just off or something, depersonalization/derealization, feel like i can't breathe/might stop breathing, moodiness, typos, etc. And for a long time I worried a lot about brain tumors. And honestly sometimes I still do, but I have to realize that after a year of this, where I have good periods and bad periods (bad periods often occurring during times of great stress), it's not likely to be a tumor. Tumors just get worse - they don't fluctuate like that without treatment. So when you get these feelings, try to take a few good breaths, let yourself relax and calm down, and know that in the process of doing so, you're offering pretty good evidence to yourself that you are dealing with anxiety. :)