What could this flashing be?

I was getting my hair dyed tonight, and i went to the bathroom to wash it out, but before i started i could visible see (with my eyes open) these flashing lights - like the flashing of fireflies - for about twenty seconds, right in front of me, and all around, everywhere i looked. It stopped after about 20 seconds. I don't have a headache, and nothing else happened. I recently got my eyes checked and the eye doc said my eyes were healthy, so i don't know why that happened. Does anyone know what it might be.