What can cause left arm pain from shoulder down to hand? not sharp pain..more like an achy pain? 29f. healthy. worried about stroke or heart attack

Probably a nerve: You are correct that heart attack can cause left arm pain, but not likely with you. You should start with an MRI of the cervical spine (neck) to look for disc bulge or rupture causing pressure on a nerve. If this is negative, the shoulder should be examined. If the pain is episodic, then a cardiac cause should be investigated by your primary doctor.

Nerve pain likely: Many things -- most likely compression of a nerve in your thoracic outlet or neck. Unlikely to be heart disease in a healthy person of 29. Definitely not a stroke. More likely thoracic outlet syndrome, or cervical nerve compression, or compression of your nerve by some other mechanism like a tumor -- see a doctor.