When you become a mother, lots of things happen. I know when I became one, I often would check on my son throughout the night, making sure he was safe and secure. Happy and asleep. You worry, obsess, and love them more then life itself.
With that being said, I can certainly understand your fears, and worry. However, when I have had similar thoughts, it is often after hearing something terrible that happened to a child. I think how terrible it is to lose a child, and I am so lucky to have a healthy, happy child. I don't think that I should enjoy the moment because they will die..type of thought. Their is a difference.
This fear will become an obsession if you dwell on it too much. It will start to interfere with your life, and your relationship with your child. If you continue having these thoughts, and feelings, I would talk to your Doctor.
While it is normal to worry and fear death in general, to continually have thoughts like this, it is a problem.
P.S. You aren't crazy..You are a mother, a very worried mommy. That happens to the best of us!