Comet with a swollen mouth

Hi everyone

I'm new to goldfish-keeping (experience of about 6-8 weeks, and a lot of
reading before that!) and would appreciate some advice on what may be wrong
with one of my fish.

First the background:
We have a 54 litre tank (with filter) and 1 "ordinary" (but very cute)
goldfish, 1 shubunkin and 1 comet.
They get along fine, although the comet gets a little rowdy sometimes and
chases the shubunkin around the tank! (I haven't ever seen him try to hurt
him though).
They are generally fed flakes & pellets, with daphnia or tubifex once or
twice a week and the occasional piece of lettuce/cucumber etc.
I check the water quality every week or two and it's fine.
There are live plants, 3-5cm of gravel and a piece of driftwood (bought at
the pet shop) in the tank.

A couple of weeks ago, the comet - Colin - got white spot. I treated it
straight away with an over the counter cure and it cleared up - the other 2
fish didn't catch it, and there has been no re-occurrence.

The problem I now have is that Colin has developed a swelling on/underneath
his mouth. I noticed about 4 days ago that his mouth looked a bit red on
one side, but the next day it looked a little better and I thought it must
have been a scratch of some sort which was healing up. Since then, however,
it has become swollen and looks like an ulcer or some kind of sore has
developed. He's still active and eating, but I'm very worried about him and
would really appreciate any thoughts on what the problem may be and how I
can help him.

Thanks in advance
Roo