I am suspected of having pericarditis and have learned that physical activity may not hurt me except that it may be painful. is this true?

Cardiologist : I tell patients all the time "it might hurt you but you won't hurt it". If your cardiologist is saying that, then let your body tell you your limit.

Partially true: Pericarditis can be very painful. Isolated pericarditis, without affecting the heart muscle (myocarditis or myopericarditis) is usually benign. So if cardiac anzymes are normal, meaning there is no heart muscle involvment, then exercise, if tolerated, is probably safe. Best, however, to rest for a few days while the pericarditis resolves.