There are several possible reasons why a person’s tailbone, or coccyx, may hurt after doing sit-ups. Here are some potential causes:

1. Coccydynia: Coccydynia is a condition that causes pain in the tailbone area. It can be caused by a variety of factors, including injury, inflammation, or degeneration of the tailbone. Sit-ups can place strain on the tailbone, which may exacerbate pain if a person has coccydynia.
2. Poor form: If a person’s form is incorrect when doing sit-ups, it can put unnecessary strain on the tailbone. For example, if the person arches their back too much or does not keep their feet anchored to the ground, it can put pressure on the tailbone and cause discomfort or pain.
3. Other injuries: A person may have an injury or condition that is causing pain in the tailbone area, such as a muscle strain, pinched nerve, or spinal problem. Sit-ups may exacerbate this pain if the movement puts pressure on the injured area.
4. Pregnancy: During pregnancy, the weight of the growing baby can put extra pressure on the tailbone, which can cause discomfort or pain. Sit-ups can also put additional strain on the tailbone, which may exacerbate pregnancy-related pain.
5. Other medical conditions: There are many other medical conditions that can cause tailbone pain, including arthritis, osteoporosis, and tumors. If a person is experiencing tailbone pain after doing sit-ups and they have other symptoms or have a history of medical problems, it is important to consult with a healthcare provider to determine the cause of their symptoms.

In conclusion, tailbone pain after doing sit-ups can be caused by coccydynia, poor form, other injuries, pregnancy, or other medical conditions. If a person is experiencing tailbone pain after doing sit-ups and they are concerned, it is important to consult with a healthcare provider to determine the cause of their symptoms and to receive appropriate treatment.