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, headache, nausea
, loss of appetite and weight are suggestive of an infection - most likely a chronic infections. Muscle ache / myositis
causing pain can be an associated symptom.
Though all your symptoms are clearly suggesting an infection, it's not easy to name the infection without a detailed evaluation. We will need to test your blood, urine, lungs and so on through laboratory test. Those will provide us clues to identify the infection.
In all means, it is advisable that you consult your healthcare provider. The aforementioned tests need to be done and treatment needs to be initiated based on your general condition and symptoms.
Hope I answered your query.
Wish you a speedy recovery