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After any major surgery that involves general anesthesia some chest infection, cough and phlegm is expected. Some chest pain too is expected. Usually taking steam and some expectorant helps.

However you must consult her doctor once because the pain can be due to pulmonary embolism too which is movement of a clot or air bubble that blocks the blood vessel in lungs. An X-ray of chest should be done to rule out infection. Pulmonary embolism if suspected would be detected by a CT scan.

Since a confirmed diagnosis cannot be given on net please discuss this with your mother’s doctor once. Hope this helps. Take care!