Hi Sarah thanks for your question
You have been diagnosed as a case of gastritis and now you’re having stabbing pain
under your left breast
Your blood test
and chest X-ray is normal.
Your unable to differentiate between pain of gastritis and cardiac pain(pain originating from heart)
One important thing in history you have not mentioned is your age
What about you’re ECG?.
Reflux esophagitis(esophagitis caused by reflux of gastric content into esophagus
) and hiatus hernia
(herniation of stomach into chest through esophageal opening) can cause stabbing pain under your left breast.
The other condition which can cause pain under your left breast is of course any heart problem.
Get your ECG and echocardiography
done, to differentiate gastritis pain from cardiac pain.
Consult some physician who will help you in establishing the exact cause of your pain under your left breast.
Hope I answered your question.