Your system might have "learned" to tolerate acutely high levels of CO2 during training, but it is not normal during rest. As your cardiovascular fitness increases, you become more efficient at using O2 and excreting CO2, and as such your ventilation rate falls to maintain a normal blood PH.
There are many reasons for an increased CO2, many of which should be examined.
A ventilation perfusion mismatch can be caused by bronchitis, asthma, pulmonary edema, hepatopulmonary syndrome. In case you are a smoker, it can be due do emphysema or COPD. Infections such as TB. Almost any pathological process in the lungs can increase the CO2.
I'm assuming your other values are fine since you did not mention them. That includes the blood PH. Your ventilation rate can decrease to compensate for a high PH caused by kidney problems; compensated metabolic alkalosis. This has many reasons, with the most frequent one being diabetic nephropathy in case you are diabetic. You can also have overtraining syndrome, which increases cortisol, which in term damages the kidneys, but that is hard to diagnose.
I would suggest you go to your doctor to check your kidney function (urinalysis and bloodwork), and pulmonary status (spirometry etc.). In case they turn up negative, try lowering the intensity of your workouts for some time to decrease cortisol levels. It is obviously a chronic change, so there is no need to worry about dropping dead at practice, however, I would recommend that you stop practicing until you know the etiology so as not to stress your body.