I can confirm the exact cause of your symptoms only after a complete history and clinical examination. There are several causes which can lead to these symptoms. A pulled muscle, trapped wind, ovarian cyst, Crohn’s disease, ulcerative colitis, constipation, duodenal ulcers, appendicitis, cholecystitis, kidney stones, cholecystitis all can pain on the right side of stomach. Since it has been persisting since such a long time it is best to get investigated. A complete evaluation and investigations like ultrasound abdomen may help in detecting the exact cause. Do keep us posted.
Best of luck and regards!