Just a thought .... Did you perhaps bring any new plants into the house recently? Quite often, fungus gnats are smuggled in when you purchase a new plant (almost all the stores have them and they multiply rapidly).
If you think this might be the case, go and look closely at the surface of the soil and see if there are any teeny tiny wiggling larvae moving around. If you see any movement, BINGO, you have your answer.
They're quite easy to get rid of but you need to treat ALL the plants in the house because once they've reached the "flying" stage they travel around to uninfected plants and lay eggs in the soil. You can purchase a white powder to sprinkle on the soil of each plant and then water in. It may take several treatments ... just follow the directions on the container.
Hope this helps!
Leah