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Before assuming this is a behavioral problem I think it best to make sure that it is not a medical condition, especially in light of possible weight loss. Here are some possibilties for medical conditions:
There are other possibilities as well but they are less common. Bloodwork and an urinalysis are the first step to diagnosing the problem. If everything looks good on the tests then you can probably assume this is a behavioral problem. If you have more questions please let me know.