For As Long As I Can Remember I Have Always

For as long as I can remember I have always had issues with going to the toilet, I can go max about 3-4 weeks without passing stools. I have tried everything to dieting to exercise to laxatives. I have only been tested for coeliac disease. I have always had many anal assessments and a flexible sigmodscopy. They have found nothing. I suffer from severe bloating, cramps, constipation and mucus. Any idea what it could be?