That sounds a lot like disassociation to me. It's a natural coping mechanism for when we're stressed out. We don't do it intentionally; it's all subconscious.

I've had it happen to me before. The first time it happened was in my freshman year of high school during Spanish class. I was paying attention during class when all of a sudden I completely forgot what I was doing and how I got there. I literally had no memory of what happened, no matter how much I tried to "remember". I didn't even remember walking to class or coming to school. It was that bad. Of course, I knew who I was and I was able to figure out I was in Spanish class, I just didn't remember getting asked a question, nor did I have any memory of what we were doing. I completely disassociated. My teacher kept pestering me to "answer the question". Perplexed, I asked, "What question?" Then she said, "Weren't you paying attention? I know you were. So, answer the question." I said, "I literally don't know." "Just answer the question." "But I don't know what the question was!" "Just answer it."

In extreme cases of disassociation, you get disorders like Disassociate Identity Disorder. People essentially disassociate their identities to cope with traumatic, stressful events -- for example, childhood abuse.