more and more frequent premature atrial contractions with DOE in an otherwise healthy 22 year old

I'm an otherwise healthy 22 year old with more and more frequent palpitations and significant shortness of breath with light exercise - when even walking on a flat surface. (Many have suggested this before, but the shortness of breath and palpitations/fluttering do not relate to periods of stress/anxiety. The shortness of breath only happens on exertion, and the palpitations - which are premature atrial contractions - happen randomly.)

Does anyone have an explanation for this?

Signs and Symptoms:
-during light physical activity (walk test) HR rises up to 180 bpm, Respirations 48 (and I'm not out of shape because I'm active)
-more frequent atrial contractions from 17 years old, once a month, to 22 years old, everyday

Right Heart Cath:

pre-exercise
R atrial mean pressure: 5 mmHg
RV: 25/6
PA: 24/9 with a mean of 18
PCWP mean: 10
High SVC sat: 79%
Low SVC sat: 80%
High IVC sat: 78%
High right atrium: 82%
mid right atrium: 82%
low right atrium: 83%
RV saturation: 83%
PA sat: 84%
Peripheral artery sat: 98

Cardiac output by Fick: 5.7 L/min with cardiac index of 3.8 L/min/m2
cardiac output by thermodilution: 6.8 L/min with CI of 4.6 L/min/m2
Pulmonary vascular resistance: 1.4 wood units

cath with exercise:
baseline PA pressure initiation: 25/11 with mean of 19
peek PA: 40/10 with mean of 21
PA sat: 49%
periperhal sat: 100%
cardiac output by thermodilution: 10.1 L/min, CI of 6.8 L/min/m2


echocardiogram is normal. BNP is normal. thyroid is normal.

I also had pulmonary tests too, but I can't remember them.  They were normal.