24 F with pain under left breast and to the side, maybe in ribs, tender to touch. More pain when slouching. Been going on a year. What could it be?

Costochondritis. This sounds like costochondritis, a form of rheumatism that is very common in young women especially. NSAIDs like ibuprofen (400-600 mg with food every 4 to 6 hours) or Aleve (naproxen) (1-2 tablets with food twice a day) usually helps.