Could Painful Lump On Lower Rib Cage Be A Cancerous Tumour?

Hello I have had a painful lump for a week on lower left rib cage , on top, not under rib. Painful to touch . And niggling hurting feeling when sitting. It is about 1 inch long. Feels like a vein.
Doctor dismissed it as nothing to worry about and that it has probably always been there but recent sports activity might have aggravated it. I don't agree and am worried about it. Could it be a cancerous tumour?