Possible Malabsorption. Should be done.

Hi, I am suffering from chronic headaches due to possible Malabsorption of food. These headaches occur within 2-3 hours of having a meal and the headache does not ease/subside until I don't clear my bowel. Due to this, I need to clear my bowels atleast 3 times a day. Also, because of this I wake up with an extremely heavy headache. It almost feels that whatever I eat is rotting inside my body and until I don't clear my bowel, the headache remains. I have shown to a gastroentologist who got my stool,blood tests done that were absolutely normal. He also suggested endoscopy and colonoscopy which I didn't get it done because I felt he was going nowhere with the treatment and was just doing guesswork. Taking isabghol helps a bit, I have done treatments from other doctors as well like taking nutrolin b plus,metrogyl,vimliv but they have not been much effective. Kindly suggest and if possible provide a detailed explanation of what should be done.