Pain inbetween shoulders blades and pain under right rib when sitting down. is this gallstones?

Rare, but possible: Gallstones in a 16 yr old are rare, but not impossible. Pain between the shoulder blades and under the ribs is more likely from a muskulosekeletal problem than from gallstones. People with gallstones often have attacks of severe abdominal and right upper abdominal pain, plus aversion to (or they get nauseated by) foods that are rich in fats.