my 4 year old daughter is all of a sudden chewing on her clothes and her barbies hands and feet, my mother in law says that there is a disease out there that she could have that makes her feel as thought she has to chew on things. I've never heard of it and I am not sure if it is a real thing.
If she means something like Pica...I don't really think it is. Pica is a disorder that causes the person to actually eat things that aren't what we consider food. There is another disorder in which a person cannot stop eating unless held down, but it's very rare, and you said that she isn't actually eat it. So you can probably rule out an eating disorder.
You can check to see if she is deficient in some vitamin.
Other than that, has there been a reasonably big life change? She could be letting out anxiety about some situation by chewing on things, or it could be that she's just bored and discovered the texture and fun of chewing on things like that.