I've only seen this happen for two reasons, when a storm is approaching and when there is something wrong with the food. I've even had all of my tortoises refuse to eat because there was something wrong with the food. Then again I do give all my pets high quality food.

I suggest refreshing the food you offer and ensuring its as high quality and fresh as possible. I do this and every time I help family out with their reptiles the health and condition of the reptiles surprises them.