I've 2 or 3 times also undergo such a shiny effect on my hero ... it's very strange ... I think I was in Broken Valley and killing Goblins nearby the West Valley Scrine, but I can't remember if I was using a fireball skill or not ...

I always "saved" my game (to be sure I make a seperate save) and after the saving the shiny effect was gone.

On 7th of february 2015 : I start a new adventure in the Divinity world of Original Sin,
it's a Fantastic Freaking Fabulous Funny ... it's my All Time Favorite One !