You have not described the characteristics of the pain. I am not sure what you call a "problematical object".
Normally a pacemaker, properly implanted, once the incision has healed, should cause no pain. Certainly no pain when walking or perfroming daily tasks. Most assuredly there should not be "much chest pain". If there is pain, this should be discussed with your physician as soon as possible. It is possible the pacemaker is shifting under the flesh, which should not be happening. It is also possible the pain is cardiogenic in origin, which cannot be determined from the information posted.
In any event, do not let this go, but bring the situation to the attention of your physician as soon as possible.
It happens in some exercises. Exercise more, but this time do gentle exercises that allow the body to rebound in the exercise. If one is still concerned, one can always find a way to do a specific test.