Seromas are common, generally mild complications that can follow any type of surgery. They occur when fluid accumulates underneath the skin in the area of the incision. Most seromas resolve within two or three weeks, and they usually don’t require any major treatment.
If your vet re-evlauated Hae Yu and determined that the seroma had healed, then I doubt that it is playing any role in the current situation.
What is most likely is that Hae Yu is developing fat deposits on her abdomen. These are officially called abdominal fat pads, but some folks prefer terms such as “Buddha belly”, “beer belly” (which isn’t really appropriate since most cats shun beer) or “dangling participle”.
These fat deposits occur primarily in spayed and neutered cats. They occur on the rear portion of the abdomen. Cats do not have to be overweight for them to develop. They do not form in all cats, but in some cats they are pronounced. One patient of mine, a Sphynx cat, was completely hairless and developed an exceptionally prominent pair of fat pads on his abdomen after he was neutered. The pads were unsightly, but they were harmless.
And that is the key thing to remember. Abdominal fat pads are, in general, harmless. You might want to have your vet take a look at Hae Yu to confirm that nothing else is going on. But my hunch is that there isn’t much to worry about.