Did the belly just swell suddenly, and quickly, there may be something she ate, causing discharge and discomfort. If belly has been swelling for a long period of time, and cat not fixed, possible pregnancy. Keep an eye on her. If she starts to get worse, call vet, there should be an emergency number. Or check with local pet shop, a lot of the employees can help you if you can have a one on one conversation with someone to better describe what is going on