White Mist

I live in an almost 300 years old farm. My house was built by monks. I have always felt weird stuff in my house, like the occasional cold spot and noises, but I always just thought it was in my head. A few years back when I shared a room downstairs with my sister (the room I am currently writing this story in) I saw something. It was a boiling hot night and I was desperate for a drink. I sat up and grabbed my drink, when I took my first sip, I saw something at the bottom of my room and I was petrified. It wasn't any particular shape just like a really clear white mist with a strange movement in the middle going outwards. It is so hard to explain. I slowly put my drink back and slipped under the covers and didn't look back up. My family didn't believe me, so I invited a friend around to sleep, not telling her about what I had seen. That night she woke me up crying, saying she couldn't sleep on her own, so we moved into the living room. The next morning she told me that she had seen something very similar to what I had seen, but this time, it moved out into the hallway and it was gliding very slowly. I don't know what it is and I hope that you believe me because I wouldn't lie about this sort of thing, if you have any ideas it would be very highly appreciated.