Take it out, have a cuppa and relax.
What do you mean by a huge tummy? Has this happened since you last saw her or has been swelling up for a few days or was she pregnant?
When you say you are scared of a plague do you mean other fish are looking sick or you hope that whatever killed this fish isn't contagious.
Often livebearer females who are heavily pregnant become totally exhausted due to being chased by more aggressive fish looking for a free meal of newborn fry.
Healthy fish don't just pop their clogs out of boredom so if you can then post a picture of her, your tank and a list of inhabitants along with the water parameters (pH, KH, ammonia etc)