Brief Answer:
Need examination and investigation

Detailed Answer:
Hi friend
Welcome to Health Care Magic

     Some conditions need evaluation – it may be inadequate pumping from the heart; it may be fluid accumulation in the lungs – heart failure (this is the most common cause) / progression of the original disease process or new process in another area / complications of surgery – inability to eradicate the original cause, inadequate muscle or blood supply / infection etc. etc. / or incidental unrelated events
     The treating doctor alone could conclude better based on total clinical picture…      Physical examination and investigations like EKG, ECHOcardiogram, blood tests are a must for further assessment and assistance

Good luck
God bless you

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Brief Answer:
Likely drug induced ...

Detailed Answer:
Hi

Possibly from the medicines / especially aspirin given after heart surgery
Several others - food, gastritis, ulcer, hiatus hernia etc.
An Upper Gastro-Intestinal-Endoscopy (‘Gastroscopy’) can clinch the diagnosis

In the meantime, Over The Counter (OTC) antacid like magaldrate / Proton Pump Inhibitor like Omeprazole or H2 antagonist like ranitidine / Prokinetic agents like Domperidone – can help

Regards

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