Is it normal that i have a lack of emotion

Ever since I was little, I have always lacked emotion. Whenever someone was happy or sad I could never sympathize with or share what they felt. Everyone thought I was strange for being this way though I never understood why. Everyone else was always so exited to get a reward for doing well, but I didn't care at all. When I was older, in middle school, my friend was talking about how incredible she felt, but when she asked how I felt I said that I didn't feel anything. She was confused at this answer. Again, I never understood why. When I found out that someone I knew was sick, everyone was concerned, but I wasn't. I frankly didn't care. Once, when a family friend's baby was in pain, I couldn't sympathize with her. When we, as a tribute to 9/11, watched a video of it from the kids point of view, everyone was so sad (almost heartbroken). I, again, was just sitting there, thinking about how cool the explosions looked. I know it's probably not (judging by everyone else's reactions to my lack of feelings), but, is this normal?