Welcome to Health Care Magic
You have not said anything - even gender and age / other disease or risk factor - to make proper inference.
Left arm pain
with breathlessness? It points to the HEART.
You MUST GO TO THE DOCTOR NOW - an ECG (Electrocardiogram) and cardiac markers - probably an ECHOcardiogram
and X-ray chest - need to be done. If nothing is found, cervical spondylosis
can be investigated leisurely!
In suspected heart disease
, TIME IS OF THE ESSENCE – if detected early, an attack may even be aborted / the amount of damage and complications can be reduced remarkably. The most important time is the first few hours.
You MUST GO THE EMERGENCY DEPARTMENT - IMMEDIATELY
Better be safe than sorry - treated if a problem is found / Happy, if it turns out otherwise
Wishing you speedy recovery