Trileptal and Low Sodium

I take trileptal 1800mg/day. I got routine labs drawn yesterday to monitor my sodium level. I’ve been on trileptal for a very long time and have never had a problem with low sodium. The doctor called and told me my sodium level was 126 and very low. He told me to go on a strict fluid restriction allowing only 1 liter per day…this is very hard because all the meds I take cause dry mouth. I am training hard also for the RACE TO SAVE FACE in July so I sweat a lot. Does anyone have any suggestions or ever had low sodium levels on trileptal. I value all of your suggestions!! I came to my TN family first!!