thanks for sharing your concern.
The reason being that either your palate
expander is causing impingement on that particular tooth or there could be gingival (GUM) recession and root exposure on that particular tooth.
Do you have this problem in your anterior(front) teeth or posteriors(back tooth)?As the canines and premolars,have long roots and sometimes gets exposed due to gingival gum) recession.
i would suggest you for a revisit to your dentist
and get it examined for any tissue impingement or recession of gums.further it can be managed accordingly.
hope this helps.