I have ibs-c. i'm on miralax (polyethylene glycol) plus i consume 30 grams of fiber everyday. sometimes my bm is pencil thin & other times it's normal. is this concerning?

Not a concern: narrowing of the stool is generally not a serious matter in a 20 year old, especially if you have IBS. some cases may also be due to narrowing or spasm of the anal canal, so evaluation by a specialist may be helpful in ruling that out.

IBS-C: it is not unusual to have different appearance of bowel movements randomly. At the age of 20, these kind of changes are inconsequential. Unless you see red blood or black tarry stools, this is of little concern. I agree with continuing a high-fiber diet. Let your doctor know if there are major changes to your bowel patterns. Good luck.