Something unusual happened to me last night. I woke up in the middle of the night and as I was walking around the house, I began to feel very lightheaded, like I was going to pass out. It was similar to the feeling you get when you stand up too fast sometimes. I started breathing heavy and broke out in a cold sweat. I fell onto my knees and held onto a chair. then I climbed up onto the chair and sat in it until it passed. Then I got up again, slowly and I was still feeling really lightheaded. I went outside for some fresh air, and I felt so weak I had to lay on the front step. When I felt strong enough again I made my way back to bed, still in a cold sweat, and almost immediately passed out. I felt fine when I woke up and I still feel fine. What could have caused this?