Change in stool caliber and consistency can be due to a narrowing in the anus or rectum. Patients who had haemorrhoid surgery done may develop narrowing of the anus and pass narrow caliber stools.
The most sinister cause would be a narrowing of the rectum or descending colon due to an inflammatory or malignant process.
In older patients, these symptoms should never be taken lightly and a diagnostic colonoscopy would be the obvious next step.
At 25 and assuming no family history the odds of it being cancer is low, but not zero. You should see your family GP and have the doctor perform a rectal examination for you, and if necessary refer you on to a specialist for further evaluation.
