Lump covering eye, please advise!

Dear All,
    I hope someone can help me, I have a female goldfish, who has now been
with me for 5 years. She has recently developed what first of all seemed to
be a cloud over her eye, has now developed into a very substanional lump
over her eye. I'm worried that it maybe some kind of cancerous condition.
Twice now when I have cleaned her out, when she has been placed into
different water the lump has started oozing blood from different parts of
it, and the blood has appeared very 'bitty' but perhaps this is just me
thinking this as it's in the water.

The lump is very noticeable, but the other eye has not been affected at all.
Also it doesn't actually seem to be affecting her, she remains active and
still has a good appetite. Its a real worry and would appreciate some
advise. She lives in a large tank, with just water in (no pumps or anything)
she is cleaned out every 2 weeks or so and the water is treated with a
number of disease treatments, though have always been treating her with
these since fin rot about 4 years ago.

Please help!

Christopher Lewis