Sharp pain behind left breast, on and off. could it be my acid reflux? cardiologist cleared me of heart disease.

Yes: It could be acid reflux/esophagitis. If it is very sharp and increased with deep breaths, it could be a temporary inflammation where your ribs meet your breast bone...Or something more serious. If the cardiologist "cleared" you, you most likely do not have this more serious condition- a blood clot in the lung. Your condition is probably innocent and should just be observed for changes or resolutio.