Why would my o2 levels drop from 98% to 83% when I'm resting?

Medical Attention... You really should seek medical attention. Assuming the device you are reading your saturations on is accurate this could be the sign of cardiac or pulmonary problems. Sometimes when oxygen levels fall at rest the device (pulse oximeter) that reads the saturations is erroneous (ie needing a new battery, calibration, etc). I would advise seeing your health care provider.