What is wrong with me? I am losing an average of about half a pound a week for the last almost two years...?

Sometimes it slows down sometimes it speeds up. If I don't consciously try to prevent it then it speeds up. I wasn't officially overweight before but was high normal; now I guess I am low normal. I am skinny compared to most people. So losing weight was okay, But since I haven't tried to do lose weight it seems strange that it is happening.