Hi,
Your grandmother might be suffering from either a heart problem like congestive heart failure or possibly a lung disease. Since it’s a sudden problem and not a gradual one, it’s more likely a heart problem as the lung problems usually develop gradually. I’ll suggest an echocardiography to find out the status of the functioning of her heart which should definitely help. An echo is able to find out not only the functioning (ejection fraction) of the heart but also how the valves, walls of the heart are functioning.
I sincerely hope that helps. Please do keep me posted. Kind regards.
Most likely she has heart failure or COPD which creates breathing problems. Ask her doctors what her diagnosis is or order her medical records.