According to the only symptom you report (“pain on the left side of the stomach under the ribcage”), it is impossible to identify the cause of the condition withoutfurther diagnostic procedures. First you will need a standard physical examination (inspection and palpation). Then you can request standard blood/urine analyses, plus liver and kidney status, and an abdominal ultrasound. Assuming a possible correlation in the upper GIT (esophagus, stomach, and duodenum) you will need then require a gastroscopy. Further diagnostic procedures will depend upon results of the previous examinations. You can ask for an expert opinion from a gastro-intestinal internist.