Why do I feel this way? (please see details)

I don’t know how to describe it.
I used to be so happy, joyful and optimistic. Now I’m completely emotionless. I can’t cry at anything (except for things about animal abuse), it’s like NOTHING phases me. I literally have to force myself to smile and laugh. I haven’t laughed a real laugh in about two months. I also feel like I have absolutely no motivation, and that life is passing right by me. People think I’m cold and heartless if I don’t express the same emotions as they do.
So, what the heck is wrong with me?