Gallbladder cancer causes a chronic dull ache, or a dragging sensation in the upper right abdominal area, instead of the typical sharp pain from gallstones. The sensation persists as a steady feeling that might occur at intervals, and possibly spreads across the upper back and shoulder blades as the cancer progresses. The dull abdominal ache may not be too bothersome (and less painful than gallstones), and hence, people may dismiss it easily, when the condition is still in its early stages.
Sources
Cancer Research UK
American Cancer Society
National Cancer Institute (NCI)
Disclaimer: This article is only for information, and is not a substitute for medical advice. Always consult a doctor if you notice any symptoms