Green Light

This experience has happened about two years ago, when I was in the 6th grade. I am now in the 8th grade so this experience isn't all too vivid in my memory.

It was on a Sunday night around 11:15 p.m. I had sleepily gotten out of my bed to get some water. Since I have always been petrified of the dark I turned on every light in the house. After I had drank some water I decided to go downstairs to my dad's room in the basement.
Ok I should explain to you what my basement looks like: As soon as you go down the stairs straight ahead is the exercise room, on the left side is the laundry room, and on the right side is my dad's room.
So as I was walking downstairs in the dark, (since my stupid self didn't feel like turning on the light) I stopped at least 4 steps away from the ground. Something told me to look up, as I looked up in the exercise room I saw this green light floating (the light was about the size of a light bulb). At first I thought it was my neighbors shining a flashlight through the window (take note that I was still sleepy). Then I had began to realize that it couldn't be possible, because we had a tall fence that you can't climb over, seperating us from the neighbours.
Fear began to rise up in my body, and I became a little light-headed. I jumped those last few steps and ran into my dad's room. I begged him to let me sleep with him, but he said no. So I ran upstairs without looking back and into my room.
After a while I fell asleep and never went down into the basement at 11:15 p.m for awhile.
I haven't seen the green light again since then, but it still kind of creeps me out thinking about it.
I would like to know what I had encountered.