My Sister Just Had Surgery On Her Thyroid For Benign

My sister just had surgery on her thyroid for benign tumors. They removed part of her thyroid which was pressing on her trachea and esophagus. She is also diabetic. Today at the hospital she showed me a mass on her back in the lower right flank area. It is about fist-sized, feels soft and almost like it is fluid-filled. She revealed that it has been there for about a year, but she hasn't said anything until now. Someone from the hospital was supposed to check it out. Could this be related to her thyroid problem?