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The antibiotics may have completely cleared up a chronic sinus infection that you didn't even know you had until you got sick. That would explain your perception that you are breathing better now than before your upper respiratory infection.
Of course, you know that smoking is terribly bad for you, and that you should stop. Really, nothing you smoke is good for you. Within a very short time, your body will begin to feel better in so may different ways. Please see this link for a timetable of what to expect when you quit: