I had a very disturbing dream.....?

I dreamed that my husband was shot & killed at work the other night. The dream was so real that I woke up sweating, crying hysterically and couldn't imagine what had just happened. I had to get up from bed & call him at work just to make sure it didn't really happen. Can anyone explain why I'm constantly having this dream? This has been going on for some time now. It is starting to scare me.