It's not a stupid question at all.  There is something called "paraneoplastic syndrome" which is when a tumor produces proteins similar to those secreted by the brain.  Problems start when the immune system not only starts attacking the tumor, but the brain as well.  This is extremely rare and the syndrome often starts before the tumor is found.

Now, getting back to your situation....  I'm not familiar with your history and don't know how long you've been having seizures.  If it's been for several years, I wouldn't think the breast lump is related.

I fully understand your anxiety about all this.  I've been through 3 cancer scares in as many years (breast, colon, and thyroid - which still isnt' resolved yet).  The waiting for answers is nervewracking.  There's no point in telling you not to worry because you're going to, no matter what.  I can only suggest trying to distract yourself with whatever will work until you can see the doc and find out what's really going on.

When are you seeing the doc about your breast lump?  Keep us posted, k?

Sending luck and mojo. :)