I have a bone right inside the top of my anus and it makes it difficult to do sit ups. ?

This may be muscle: This may be muscle pain or bursitis pain in that location. These can attach to the bone. You can see your physician to have this checked out. On occasion we have to have you see a gastroenterologist or you be going to imaging. But most of this will depend on the physical examination

Tailbone: this is your tailbone or Coccyx. In some people it is positioned in a way that gives them discomfort if they sit on it or sit a certain way. Using a padded sit up chair or what is known as a doughnut cushion if you were doing sit ups on the floor might help. Hope this was helpful. Dr. R