I'm kind of getting creeped out! What's wrong with me?

Every time something bad happens to me that I should feel sad, I don't. My best friend moved away last week, and I cried while she was leaving(because I am a very sensitive person) but after she was gone, I didn't feel sad. I just felt...numb, kind of. It's like I am in such disbelief that my mind can't process the fact that it's actually happening. A bunch of other sad things have happened too. but I feel the same way. I try to use this numbness to my advantage and do fun things to be happy, but it seems that when I get my emotions back, they go downhill and I feel sad again. So what is wrong with me? All this is happening at a very stressful time for me, too, could that be the reason?