Why does mild pericarditis cause increased heart rate (86-98) at rest and breathlessness? i am taking ibuprofen and colchicine?

Simply put : The heart is inflamed, and usually a small amount of fluid around the heart. The heart has to work. Bit harder to push the same amount of blood throughout your body. To do this task it beats a bit faster.

Pump restriction?: 'mild' pericarditis, controlled for the moment with anti inflammatory meds such as you are on, shouldn't cause many symptoms. I would see your doctor/cardiologist and have this rechecked. A lot of the answer depends upon what is defined as 'mild', how recent the problem began and a few other things that testing with an echocardiogram might answer.

Generally due to : Pain....Unless constriction is present which is highly unusual.