Brief Answer:
Liver disease possible.

Detailed Answer:
I read your question carefully and I am sorry about the symptoms you are experiencing.

Dark urine at times can be related to certain foods like fava beans or rhubarb, or drugs like metronidazol, nitrofurantoin, cascara or senna containing laxatives etc.

On the other hand, if you have taken no such food or drugs, it could be a manifestation of liver disease, kidney disease or muscle damage. Nausea could accompany all those conditions especially liver disease, headache as well.

So you should be checked out, apart from being physically examined for other manifestations (skin jaundice, liver enlargement etc) also with some lab tests for liver and kidney function, complete blood count, creatines kinase (muscle enzyme).

Further management will depend on the findings. You shouldn't postpone seeing a doctor as these are serious conditions.

I remain at your disposal for further questions.

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