Arachnoiditis occurs when the arachnoid membrane that surrounds the nerve rootlets becomes inflamed and the roots “stick together.” It occurred more frequently with oil based myelograms (old school) but there are rare cases of that occur with the current water-based dyes. According to the radiologist, you have a single root inflamed which is an inflammatory mono-radiculopathy, not an arachnoiditis I assume this finding was present before your lumbar puncture and even a reason for the test in the first place. Contrast can make some individuals sick like the signs you were showing post-CT. More answers might come from the radiologist who is more familiar with this.
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.