I have depression and GAD and I get this as well. I lie about "not knowing" the answer to a lot of simple questions for no reason whatsoever, I forget things the moment I need them, and am terrible at expressing ideas. I thought it was my anxiety for the most part; I figured my mind was just blanking or I was trying to avoid conversation. If it isn't something you do out of fear or avoidance, though, I would say it's just in your nature. I know that anyone with a mental illness can go through changes every so often, so I don't think it is anything to worry about. You are certainly not going crazy, your mind is just trying out new things and adapting to new situations