What are some causes of unintentional weight loss?

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Question by Ducky: What are some causes of unintentional weight loss?
Can you possibly give me some causes?
Here are my symptoms:

Rapid weight loss (about 28 pounds in about 3 weeks)
Headaches
Sudden drowziness/lethargy
Joint pain.

I am seeing a physician tomorrow, but I want to know what kind of diseases or conditions could possibly cause this.
And no, I have not changed my diet or exercise routine.
I run 7 miles everyday

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