What Are The Possible Reasons For Metallic/bitter Taste In One Side Of The Mouth?

Hi. Possible reasons for metallic/bitter taste in one side of the mouth please (up the right side of the tongue and feels weird under the right side of the tongue/gum area)? Worsened over a period of about 10 months. Used to be mainly at night, but is now all the time. Dentist has checked. No obvious reason. Thanks.