How long is she doing this?
what age, breed and weight is she?
is she eating normally?
Sometimes dogs will lick certain objects if they are anemic or lacking a certain nutrient. However, your girl is very bonded to you. You are an integral part of her life. She does not even exist when you are not home. She smells your scent in the bed. She is licking the sheet the same way she would be licking you. It comforts her and makes her feel secure. The best way to stop this is to provide her with an alternative. Give her some old clothing, you no longer use. Every time she goes for the sheet - tell her NO! and give her the clothing. As long as she is not the type to actually swallow anything she will subsitute the clothes for the sheet and it will be safe for her.
Please let me know how this works out and if you still have questions.I will be happy to help.