What can be source of left side pain/flank that is worsening? radiates to rib cage (left to right) most severe when lying down neg ct w/contrast

Many things: The negative ct scan has eliminated many disorders such as kidney stones, pancreatitis, abscesses, or intra-abdominal bleeding. There are many other possibilities that ct scan of the abdomen will not see, such as spinal disorders, musculo-skeletal abnormalities, or shingles. You should be re-evaluated by your physician if symptoms persist or worsen.