In general, however, mild tachycardia (increased heart rate) is very common in pregnancy, especially in the later stages, is not dangerous and, in fact, is expected. It is usually caused by a combination of changes in blood volume distribution between the mom, placenta and baby, and a loss of capacity to increase the stroke volume of each heartbeat as the uterus rises out of the pelvis increasing intra-abdominal pressure and pushing the diaphragm up so the heart compensates the demand for increased output by beating faster.
With regard to your specific query, 65% blockage of a coronary artery is usually not enough obstruction to be blood flow limiting and typically would not be clinically significant to require correction other than interventions to prevent further progression - diet modification, exercise, control of blood pressure, glucose levels and inflammation.
Typically, chest pain of a cardiac origin is described more as pressure (‘an elephant sitting on my chest’) and can also radiate to the arm or jaw, and may be more commonly induced by increased cardiac demand like exercise, anxiety or stress. Chest pain as a result of inflammatory conditions in the chest, such as pleurisy or costochondritis, is described as sharp and stabbing, is very short lived, and is induced by situations that increase intrathoracic pressure - deep breath, cough or sneeze.
Transplant immunology - what you asked about regarding rejection - is a very complicated subject - and for solid organs - liver, kidney, pancreas, heart, lung - is a life-long risk, and although it diminishes over time, requires long-term anti-rejection medications.