Wow, great story telling. That was VERY intense lol, had the hair on the back of my neck standing up. I'm glad to hear everything is ok. I think I would have done the exact same thing in your situation, and gone downstairs to confront whatever it could have been. Although I don't have a gun...
One time when I was young, like in 4th grade, my buddy was staying at my house one night and we were up really late playing video games. We could hear my mom snoring in her room, and my dad lives in Cleveland, so thats everyone in the house accounted for. All of a sudden my buddy and I heard something outside my bedroom door, and then we both watched as the door knob turned and the door opened a few inches. After that we just sat there for a few seconds really freaked out. So i grabbed my swiss army knife and went out into the living room. The windows were closed and locked, the front door was closed and locked, there was no one anywhere. To this day I have no idea what happened, but I know for a fact that when I get my own place I am for sure investing in a firearm.