koimaster

it would have been better if you had a photo before treatment as said above yes it could be two things
1.carp pox
2 tumor

the trouble being with the redness you cant tell if its a carp pox i would go out on a limb here and say if its a small fish and the spot was white before hand it prob is a carp pox and if its a bigger fish then a tumor as big fish are less likely to get carp pox.

abit about carp pox to fill you in:
carp pox is a viral infection that will not harm you koi one little bit. sometimes it is past to weak fish to weak fish but as i said is something not to worry about. To date there is not a treatment for them apart from heating your pond to a magic number of 21 degs but this comes with a big price tag. afew companys say they have treatments for carp pox but i believe [with many others] that its just something to gain extra income as small koi get pox almost very spring. i use virisure in my pond yes it limits the amount i get but the small weaker koi still get pox.
hope this helps you out