When an individual swallows air during eating or by natural processes of digestion; gas is frequently trapped within his digestive tract. Generally, gas is not regarded as a thing to which one should worry about because it can pass either through esophagus or rectum easily. This gas which is trapped temporarily within digestive tract causes discomfort and pain until or unless it goes out from the system.
However, if a gas cause pain in lower (left as well as right) abdomen more frequently and also accompanied by some other symptoms like constipation, vomiting, excessive weight loss, heartburn and blood in stool etc. one must speak to a physician or doctor for clinical evaluation.