The left side under my rib has been hurting. what can this possibly mean?

Musculoskeletal: Not sure of the circumstances leading up to the injury, but given your age, the discomfort is likely due to musculoskeletal strain, i.e. Muscle or tendon strain. Some non-steroidal anti-inflammatory agent, such as motrin, rest and heat to the area should resolve this over the next week. If the pain persists or worsens, see your physician.