It's being a challange for me. Problems?

Hello i am 20 year old male. Four months ago i started feeling a lump in throat feeling and hard to burp all day. My local doctors diagnosed me with GERD i took there medicine for one month but no relief...but suddenly in just 2 days i felt normal again. But after one week of normal life i again started feeling that lump in throat and this time i also had constant urge of stool all day. Its like some pressure in my hips (heaviness). I had frequent bowel movements every day but still felt unsatisfied. I have normal stool not hard not loose. Again a treatment from gasteroenterologist my lump thing was gone but pressure all day and feeling of incomplete evacuation is there. My doctor has done all blood tests, thyroid, endoscopy, proctoscopy. All things are normal. Then he suggested to consult a psychiatrist. My psychiatrist has prescribed me sertraline. But i am not getting why ssri for stomach problems? Symptoms now: Feeling of incomplete bowel movement all day, hard to get out burp some