What is wrong with my sister?

My sister has been having these bizarre "spells" that sound like either some sort of syncope or seizure. She had a 3 day portable heart monitor and nothing showed up, her labwork was good. She is on a stable dose of Synthroid and no other medical problems. She found a post somewhere in which a girl described exactly what my sister is feeling and was never able to explain. I will copy and paste it and welcome all input!Here it is....

"My auras scare the life out of me. They just burst onto me like a really strong deja vu. I start to feel really dizzy and I get the weirdest thoughts of some ridiculous memory, that I can't understand why I should be remembering it. It could be a song, a talk with someone from years ago, or other things that I have no reason to remember. I can't talk properly while this happens and I have to try and talk myself out of it. I try to breath in through my nose and fight it as hard as I can.

I use this analogy... It's like a big black hole appears beside me and if I look into the black hole and try to see what's in there I'll fall in. If I fall in a seizure will kick in.

The after effect for me is tiredness and I just feel depressed and down because it's so hard not knowing when the next one will happen."