What is the difference between the symptoms of pericarditis (that develops rapidly over the course of an hour) and a heart attack?

Not much: Typically pericarditis pain will be worse when lying flat and better leaning forward. Pericarditis may occur while you have a viral infection or are just getting over one. Otherwise they can be similar. Often your doctor can use your risk for a heart attack (older, smoker, multiple medical conditions) to predict if one is more likely than the other.

Different: Pericarditis is worse with deep breath/cough. It hurts to shake your chest like the slap of your foot on the pavement when walking. It's worse lying on your back & better sitting forward. Heart attack can mimic this because some heart attacks have associated pericarditis, but in general, they feel different: heart attack usually is a squeezing, tight central feeling without change for hours.