Flying Fox Has Red Protruding Bottom!

Hi
Have just noticed that one of my flying foxes has a red protruding bottom area. Seems to have come on suddenly and doesn't seem to have caused any problem to the fish. Still behaving as normal.
I tested the water this morning and all is good. Any ideas what it may be?

people would probably want to know the exact water readings Sorry i can't help though

Hi

pH is 6.5 - 7
Ammonia is <0 mg/l
Nitrite is <0.3 mg/l
temp 25.5

Thanks

Here are some images of the issue ... hope this can help with some ideas as to what it is...