Many things. If this has been going on for a while - weeks to months - i'd get it checked out by your doc. It could be a muscle strain, pain in the cartilage or muscles of ribs. It could be gastritis, which is inflammation of the lining of the stomach. If you're also having fevers, weight loss, or the pain is waking you up at night, that's more concerning and should definitely have an eval by your doctor.