Pain under left rib cage and blood in stool?

See your Doctor: Question is vague. Any change in stool caliber? Any weight loss? You need to have blood work and probably a colonscopy and ct scan thorax. Do you or have you ever smoked? Do you have cough? Of course, these symptoms may be nothing but without knowing you, and you seem concerned, i would advise going to your pcp for evaluatin. Do not worry, but seek help.