Red Bump on Fin?

[Aluyasha]

Melvin, my blue marble CT, Has a bright red bump on one of his tail fin's rays. It is about the size of a fish food pellet. It seemed to just suddenly appear, it has been almost a week and it is still there.
I read somewhere that is might be a cyst or ulcer or maybe even a tumor.
Does anyone know what this might be?
I will try to get a picture, it is very hard to photograph.
Thanks.