What happened to my fish?

I was out of town for a week or so and got my room-mate to take care of my fish. I returned this morning to discover that one of my swordtails had become extremely bloated. She was so bloated that she was belly-up, her eyes were bulging out, and her scales were pushed forward, almost as if they were fuzzy. I knew she didn't have much longer to live. Does anyone have any idea on what happened to my fish? I've had cases of bloat before, but never this big...before she died I switched her into another tank so the other fish wouldn't eat her remains and pick up whatever she had. Could it have been bad food? Or something worse? And after she died I noticed orange markings on her stomach area that were not there earlier. Anyone know what happened and how I can prevent the rest of my fish from getting this?