Does holding the poop for several days cause cancer?i have done it since many years about 10.

No...but....: Stool retention not a rare condition & often starts in childhood. Some just have a slow system, others get there by voluntary by holding it, often after a painful experience with a hard or bulky poop. In time this can balloon out the large bowel & lead to problems in later life. Scheduled daily emptying & bowel wetting agents like miralax can return bowel size to normal over 1-2 yrs.