Originally Posted by CSaw
Not a medical problem. A stubborn, control problem.
Here is a link to several pages of information presented by Mayo Clinic on encopresis.

A person's willful withholding of stool, even when not due to a medical cause initially, can become a medical problem, as described on the Mayo clinic webpage of information on causes:
Originally Posted by Mayo clinic webpage, causes of encopresis
The colon stretches, ultimately affecting the nerves that signal when it's time to go to the toilet.

Each webpage of information presented by Mayo clinic on encopresis may be of interest, including preparing for your appointment.