It is common for people to feel pain or discomfort when they suck in their stomach, especially if they have not been engaging in regular abdominal muscle training. There are several possible reasons for this pain or discomfort:

1. Strain on the abdominal muscles: Sucking in the stomach involves contracting the abdominal muscles, which can be tiring and painful if the muscles are not used to being contracted in this way. People who have not been engaging in regular abdominal muscle training may experience pain when they try to suck in their stomach because their muscles are not used to being worked in this way.
2. Weak abdominal muscles: If the abdominal muscles are weak, it may be difficult for a person to suck in their stomach effectively. This can lead to strain and pain in the abdominal muscles.
3. Poor posture: Poor posture can cause the abdominal muscles to become weak and strained. If a person has poor posture, they may experience pain when trying to suck in their stomach.
4. Injuries: If a person has sustained an injury to their abdominal muscles, such as a muscle strain or tear, they may experience pain when trying to suck in their stomach.

If a person experiences pain when sucking in their stomach, it is a good idea to consult with a healthcare provider or a physical therapist. They can evaluate the cause of the pain and recommend exercises or treatment to improve abdominal muscle strength and reduce pain. Regular abdominal muscle training can help prevent pain and discomfort when sucking in the stomach.