Just a shot in the dark, your fish could be constipated (it could also be a host of other things, more details on the tank, water chemistry, type of fish, and the fish's diet would help).
If your fish has been eating less fibre than is suitable for its digestive system (this happens, for instance, when fish that are 'designed' to eat algae and plants are fed too meaty a diet) it can become constipated. If you haven't been seeing much poop and the fish is bloated, this could be the case. It's treatable - add some epsom salts to the tank, raise the temp a bit, and feed thawed frozen peas (not the outer shell, just the insides).
The fish could also have intestinal worms, or something else going on. If you provide more details, you'll probably get more answers.