Thanks for your question on Health Care Magic.
I can understand your situation and problem.
No need to worry for leukemia.
In my opinion, you should get done chest x ray to rule out
lung infection (pneumonia).
You are having cough,
chest pain on breathing (pleuritic pain) and
metallic taste in mouth, all these can be seen in pneumonia.
Pneumonia is caused by infection (bacteria or virus). Some of them can cause
knee pain and rashes.
So better to get done chest x ray.
If pneumonia is there than start antibiotics.
If still your knee pain and rashes are there than
consult dermatologist for skin lesions.
No need to worry for leukemia.
Hope I have solved your query.
I will be happy to help you further. Wish you good health. Thanks.