Pain Without Overdenture in Mouth: Thoughts?

Dr. P. asks:
A 68 yr female patient has had 4 single-piece dental implants placed in the mandible by another dentist. All four are in the anterior mandible between the mental foramen nerves. These four implants have retained an overdenture now for three years. The implant in the mandibular left first premolar site has become painful. Examination revealed lack of attached gingiva and inflammation related to the implant. There was no mobility and no evidence of resorption radiographically. The patient has an implant supported upper denture. The lower denture was relieved around the implant and a connective tissue graft placed. The attached gingiva is now healthy but the patient now reports pain only at night when the mandibular overdenture is out of the mouth. She sleeps well only when the overdenture is in the mouth. I would appreciate your comments please on this case. Thanks.