I have found, personally, that chest area pain (this includes upper stomach, arms, shoulders, neck, jaw, back) can be very distressing for those who have any reason at all to be suspicious of heart involvement. What to do when we have this kind of discomfort?
Most of the time, it's benign. It turns out to be a pulled muscle, stress and strain, it can be indigestion or gas, but always we have that nagging question in our minds. Is this something that needs further clarification or even a trip to the ER?
(Anyone who has ever spent hours in the ER is probably reluctant to go there.)
My general knowledge about pain when you take a breath is that it is not heart related. I say this with a very big caveat, that I am not a doctor, or YOUR doctor, and do not know your personal health history. However, when it hurts to expand the chest area that is usually chest wall related and indeed could be gas. Gas pains can travel up and affect us in the shoulders and make it tough to take a deep breath.
Since you had a pacemaker put in 2 months ago I assume you are in regular contact with your physician. It is certainly worth a phone call to your doctor's office, but my immediate instincts tell me that probably this is not going to be heart related. Good luck to you!