Has anyone had this feeling? (read below)?

This is an odd question that I always wanted to ask but haven't got to it. I also realize it is going to be near impossible to describe since its more of an experience, but here it goes. This usually happens when I am unhappy or depressed about something. However, this is not always the case. Sometimes I start thinking about myself. I kind of see myself from an outside view. I kind of realize what I am. It becomes very difficult to think about. It usually leads to muscle tension and brain tension. Sometimes I also get light headed from thinking about it. It has happened to me so much that I can not think about it, no matter what my mood is, if I concentrate hard enough. However, the tension still occurs. I don't think anyone will know what I am talking about at all unless it has happened to you as well. Has this happened to anyone before or am I alone?