Resting heart rate is always low but once I get up It goes 90 to 120. Rhythm was normal on holter. Could this be caused by anxiety?

Could be. Normal heart rate is between 60 to 100 but athletes often have a lower heart rate. Anxiety can cause rapid heart rate and other medical problems like hyperthyroidism , fever or some drugs. Your doctor seems to be on the right track to find out the cause.