Is my laughing at an offensive description of the Prophet, peace and blessings be upon him, considered a sin, or is it a demonic whisper intended to make me doubt my faith?

The questioner's fear and weeping indicate her hatred of disbelief and her displeasure with it. Therefore, she must completely turn away from these whispers. A Muslim, especially one who is afflicted by whisperings, should not look into the words of atheists for fear of being tempted in their religion.