I would guess that a previous fly visitor that has laid eggs somewhere in your house, which have subsequently turned into maggots then into flies which appear in your with all doors and windows closed. this particularly happens as weather warms up and the fly larvae pupate in warmer temperatures.
Hey.....as moog said they lay eggs most probably in a houseplants soil........keep an eye on these regularly during the damp, warm days or even avoid having them at all if it's really 'bugging' you, flies really are the most disgusting insects aren't they!!!!!!!!!
If you put a house fly in the freezer for 15 mins it goes into hibernation. You can take them out and place them in your hand. After about a min the heat of your hand will warm up the fly, waking it up an it wlll fly away. A neat party trick for telling people you can bring dead flies back to life!!!!