If sometimes under exertion you have it and sometimes not, and you do not have any other symptom (like chess pain) I would think that it is more probably to be anxiety or asthma related.
In any case check with your doctor.
I do not suspect heart disease, and certainly not heart failure at your age and in this case. With your old EF at 65, even with PACs, your EF is probably still within perfect range for you. Right now what I am thinking is if you have ever had a pulmonary workup done. Have you ever had a test for asthma? You may have a mild case. This is a simple test that can be done at a regular doctor visit. Allergies can also affect breathing. I would try to get your heart out of your head for now, even with that one episode you had, and make an appointment to see your general practitioner to discuss your symptoms. Even if this is a new symptom, it doesn't mean that it's not a late case of asthma or allergies, or something else going on pulmonary wise. Good luck :)