Pain in rectum sometimes after bowel movement. few times had clear stuff in toilet or loosely attached to stool with small amount of blood. causes?

Hemorrhoids : Hemorrhoids and anal fissures can cause those symptoms.

Several possible: if any bleeding then inflammatory bowel disease would fit well--a serious autoimmune disease with a strong increased risk of needing surgery and/or cancer. BUT, also possible is irritable bowel syndrome with exactly these symptoms, NO inflammation, tests all negative, colonoscopy negative. would need a colonoscopy to tell which.