You had a fusion at C5-6 and had temporary good results. I hope you did not start PT 3-5 weeks later for your neck fusion as this is early for PT.
You might not have a solid fusion of your C5-6 level. Typically, patients with a pseudoarthrosis (lack of fusion) will feel good initially but will then develop pain as the fusion fails.
A new X-ray including flexion/extension will be helpful to ascertain if this has occurred.
Donald Corenman, MD, DC is a highly-regarded spine surgeon, considered an expert in the area of neck and back pain. Trained as both a Medical Doctor and Doctor of Chiropractic, Dr. Corenman earned academic appointments as Clinical Assistant Professor and Assistant Professor of Orthopaedic Surgery at the University of Colorado Health Sciences Center, and his research on spine surgery and rehabilitation has resulted in the publication of multiple peer-reviewed articles and two books.