What JHunter has written is correct. Respiratory sinus arrhythmia is completely normal. Unless you often feel lightheaded, short of breath, etc, this is noting to worry about. It may get less noticeable as you get older.
It's thought that decreasing the heart rate as the lungs expel air which is low in oxygen and pumping at a faster rate as the lungs are filling with oxygen-rich air (so more oxygen is picked up faster) increases the efficiency of the cardiopulmonary system. Whether that's actually the case isn't known, but it makes sense to me.
Being able to see aortic pulsations in your abdomen is also normal, especially in slimmer individuals (as you seem to be). I've seen them on myself and many of may classmates on whom I've practiced auscultation/percussion etc. so don't worry too much about that either.