Hello. i have this pain under my left breast in my ribcage. to breathe in a full breath is painful. what could this be?

Likely musculoskelet: Description sounds musculoskeletal especially if it can be reproduced w palpation. Pleuritic or pericarditic pain might also be a consideration for sharp pains on inspiration. Antiinflammatories usually help musculoskeletal causes, and doctor eval with cxr/echo might be needed to eval deeper areas of inflammation in lungs/pericardium.