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Info on the z-pak.
I would go back to the ER (treating MD if possible) ASAP to let them know of your current condition. They may need to prescribe a different antibiotic. The red (blood) to your mucus needs to be checked. It may just be irritated Broch tubes but anytime ones coughs red mucus they need to have it examined and a culture (when applicable) done.
Since you state that the cough is getting worse and the z-pack is running low I would most certianly go ahead and get back to be re-treated. I would not put this off too long.
I am not a doctor. But do have a medical background from my work.
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Regards.