I m having mild pain in my lower side of stomach on the left. what could be the reasons? i have consulted the doctor as well but he said nothing .

Many reasons: The list of things that could cause left sided lower abdominal pain is long (especially in a young woman) including ovarian cysts, endometriosis, inflammatory bowel disease, intestinal spasm, irritable bowel syndrome, peptic ulcer disease, etc. I would make an appt to see your doctor again or your gynecologist for review. Many symptoms do not require extensive evaluation if they do not last.