I've already undergone appendectomy. after the surgery my belly became bigger and i gained weight. is that natural?

Not related: Your weight gain is not related to surgery , reduce caloric intake, have healthy diet , go to gym regular basis & do exercises , be active you will loose weight and abdomen will get flat.

NO: No it is not normal but what you, a 21 year old female has is difficult to assess without examination. Is the weight gain visible just on the belly or other places too. If just belly i would like to know if your belly has fluid in it or not. Would like to know what was the weight before surgery and were/are your past present food and exercise habits. See the surgeon for a follow up.