I am not aware of this side effect of Cartia sl. It is probably because of the lack of PVCs, and slowing of your heart rate causing your heart rate to drop so each beat contributed more to your cardiac output. Hence each beat seems stronger. If your echocardiogram shows normal ejection fraction, then there is virtually no risk of the PVCs causing you any heart problems.
The obstructive sleep apnea however can have very serious consequences and you absolutely need the sleep polysomnogram so the diagnosis can be formally made and treatment initiated.