I get frequent pain below the ribs immediately after i start walking. i did see a doctor and he told me that i should drink more water. i drink 3.5 l

Many possible Dx: To clarify, I assume you do not have pain when resting. But activities like walking, running, even lifting or carrying probably cause this discomfort/pain. Depending on how you walk and your usual activities, you could have musculoskeletal strain (most likely cause). Inflammation of the ribs (where muscles attach) is called costochondritis. You cd have heart disease as well, but u are rather young