Burning sensation. N much appreciated!

Hello respected Doctors, Greetings and hope you all are doing good. I am asking this on behalf of my best friend. She is 35 yrs old. Height 5.3 and weight 47 kgs. Past one year she is complaining of burning sensation n lower abdomen cramp after passing bowel and stool. Also gas burning sensation in the part just beneath the lungs. Then the pain radiates to right lower abdomen and stays there. We have done her upper and lower GI endoscopy four times. Colonoscopy,Cystoscopy and complete usg of the abdomen. There are no findings. She is not diabetic nor have any history of high or low blood pressure. Blood test for stool is negative. The color of the stool is normal. Her HB is 12. No thyroid.. Kft, lft normal. Pls advise what's happening to her. She follows a vegan diet. No sign of UTI either. Thnx n much appreciated