Do you have liver problems that you know or don't know about? It is just that ascites occurs in people with liver problems and actually means that you have a collection of fluid in your belly because your liver is malfunctioning.
If you have ascites, your belly would be very swollen as well as hard and rigid. Normally people with this have a yellow appearance too due to the liver problems. If you did have this you would have to go to the ER straight away and would have a small tube inserted into your belly to drain off this fluid.
Are you constipated at all? If you are, then this can cause a hard belly because of all the backed up stools.
A bowel obstruction may also cause this too. However, if you had a bowel obstruction, your belly would be swollen again as well as rigid and hard, you would probably vomit when you tried to eat anything and you would not be able to pass gas or open your bowels. However, this is unlikely to occur unless you have trouble with hernias or have had at least one major surgery on your belly.
Are you sure that you have not been doing too many sit ups?
Lots of people get worried by the word ascites, just because they have heard it, but are very mistaken about what it actually means. Ascities means fluid in your belly, which would be grossly swollen and hard and rigid, however you would also have others symptoms with this and would probably be feeling extremely sick.
Hope this helps?