What other than ibs or polyps or a mass can cause flattened stools at times?

Hemorrhoids, strain: Constipation or forceful straining with bowel movements without relaxation of the anal sphincter is the most common cause of flattened stools. A rectal exam should always be done to exclude and anorectal mass.

Hemorrhoids: All of the things you mentioned along with hemorrhoids can change the stool appearance. If you see changes get yourself a proper evaluation to avoid the more serious problems that can cause this.