Hi,
Noted your history about having pain under left breast, severe enough not allowing sleep, very painful on coughing or breathing deep, continues around left side under armpit.
The commonest causes I can think about are as follows:
Fracture of rib due to
sneezing or so.
Muscular strain.
Costochondritis.
This can be diagnosed by the following ways:
Clinical examination by a Doctor, preferably by a
General Surgeon.
Confirmation by X=ray for ribs, CT scan as needed.
I would advise you the following till you get the diagnosis,
Take an anti-inflammatory medicine with a painkiller property like
Ibuprofen round the clock if it suits you.
Strapping over the area to reduce the movement with respiration, straining.
Hope I have answered your query. Let me know if I can assist you further.
Regards,
Dr. T. Chandrakant, General Surgeon