We described a case with a medical history of diverticulosis and squamous cell carcinoma of the tongue who underwent treatment with pembrolizumab, carboplatin, and docetaxel who subsequently developed diarrhea and vaginal stool leakage. She was later diagnosed with diverticulitis complicated by diverticular abscess and a colovaginal fistula, in addition to collagenous colitis. Pembrolizumab was discontinued and her colitis was initially treated with vedolizumab but then changed to ustekinumab due to lack of response. Ustekinumab resulted in complete resolution of colitis and she had no further complications from her diverticulitis. Unfortunately, the patient continued to decline due to cancer progression and died 7 months following resolution of diarrhea.