A longshot (as you were out of the house), but might also be worth checking for airborne chemicals like air freshener, scented candles, paint fumes or pesticides etc being used by a neighbour that could have got into your home. These can be toxic and result in neurological damage which could also explain the seizure-type situation.
I know that when the folks in the downstairs flat decorate, I have to open all my windows as the paint fumes can be easily smelled in my flat.
As I say, a longshot but worth thinking about just in case. I'd agree with other posters that heat is your most likely cuprit.