The red gills tell me ammonia poisoning, but the body swelling doesn't. Unless that's some tricked out unknown symptom I've never heard associated. Comparing to whatever beast disease killed my guppies, yours is much slower and the red is in a different spot. The sudden body swelling does look like whatever this was. I don't remember this second if you said it was lethargic. As for the exact problem, I am clueless. My mom thought gill disease, but that didn't match when I looked it up, and I did the treatment the Internet said to do and it didn't work. I could not tell you what exactly it is if my life depended on it. I apologize.