I can definitely see it being a past life thing, though I'm not sure how quickly souls reincarnate and all that. It could even be a process out of time itself, or maybe she came back so soon (if that is the case) because she wasn't quite finished here?
It could even be a suppressed childhood trauma, like some very bad nightmare or experience as a child. I know that personally very odd things trigger memories of horrible nightmares I've had. The color green brings me back to the single most... I have no word for it... strange dream of my life. And it can trigger sickness or discomfort, for whatever reason I keep locked deep in my brain lol.
Maybe you could take her to a hypnotherapist, or get some sort of past life reading done? It would be fun and may help her heal in this life.