belly tumor or fat

My neon fish has a really swollen tummy.?

A week ago it got a funny white swelling on the edge of it’s mouth, but was still eating,swimming etc ok.
Over the course of just today it’s belly has swollen up badly.
I gather she is not pregnant, cause she wouldn’t get that fat in one day. She/he is still swimming around ok, eating fine etc, in fact it doesn’t look sick at all, but this can’t be right, can it??
Any ideas??? It has no white spots on it or any other signs of anything being wrong. Should I also remove it from the tank so all my other fish don’t get sick too? I have 10 neons in my tank, plus molly, guppy, dwarf guarami and fry. I also lost a dwarf guarami today again with no signs of being sick. I’ve tested the water and all is ok. The white tumor looking thing on the neons mouth has gone down alot over the last few days also.
Thanks for your help.

Isolate the sick fish, place it into a “hospital tank” A separate tank with lots of aeration it should benefit from the extra dissolved oxygen. This should be the first step when you recognize something is wrong with one of your fish, as for the possible diagnosis I cant help you best to ensure it is not a danger to all your fish.