Choking sensation with breathing complications

Earlier this morning while I was upsatirs I heard some laughing, but ignored it and went about my duties I do in the morning to get the kids off to school. Then I was downstairs in the diningroom and heard someone walking around upstairs, as we did many times before and thought nothing of it. Later Karen went upstairs to go to the restroom and she felt like someone was choking her and she couldn't breathe. I told her to come downstairs and go outside and get some air and she said her legs went to the ground and it felt as if someone was standing on them and still choking her. She eventually did get downstairs and I went outside with her for her to regain her composure. She went back up later on to go to the restroom again and the same thing happened to her, but without her legs going out and feeling as if someone was standing on them. I told her to not go back upstairs and I went up to look around. While I was up there I felt sick to the stomach and dizzy, but I stayed up there and found nothing, but I also said a little protection thing to try to help protect me while I was up there.

A new development with this. Karen was sitting in the livingroom by herself and she said she heard a man's voice that said, "You like what I did?" Come upstairs and i'll do it again."