I recently posted about my tiger barbs flashing. Sure enough, when I checked my water my ammonia was up.
I did some water changes and my ammonia is back to 0. Still have some flashing going on but not as much, so time will tell if that was why mine were flashing. Plus, they had no other "visible" signs of anything.
So, perhaps you should check your parameters and some water changes like paekakboyz suggested.
Caper