Is the white light lamp bulb new or one you have used before?  Some bulbs are coated and give off toxic fumes that can be fatal quickly to chicks, it is safest to use only those that are labeled for poultry.  They're not old enough for coccidiosis yet.  Does the feed smell off at all, look off at all?  If you did not empty the bag into another container I would suggest doing so and checking the entire contents for anything that looks spoiled.  It does sound like something toxic at this point.  Also using a thermometer to get the actual temp under the lamp and on the pad would be good.