Oh, ow! >.< I had detrimental effects on eyesight when taking just 200mg of yeast-selenium per day, a few years ago. I have read in other forum posts here on Iodine VWT that yeast-selenium supplements have unreacted elemental selenium in them. Elemental selenium is definately -not- a good thing to ingest, as it's converted to selenides in the body (after being metabolized by gut flora), and those are rather damaging!
Se methionine or Methyl-Se-Cysteine , meanwhile, aren't gobbled by gut flora, and are better absorbed.
I think among the Selenium cofactors are B3 (niacin) and vitamin E (mixed tocopherols and toconutrienols).
Perhaps the symptoms you experienced were related to shortcomings in the cofactors, combined with the source of the selenium (yeastiebeasties)?