The CT scan is negative. That does not mean brain mets are ruled
out..........
It is by FAR the likeliest explanantion

I'd ask about B12 and thyroid.
Anyone really can have what they call a psychotic break.
Happened to my grandmother, after she'd been living alone for a while.
We went to visit her and she was sitting there cutting up paper into little
pieces and wsan't making much sense at all.
We called my aunt (her adult daughter), consulted doctor and decided she
should be placed in a nursing home. At that time, I never thought to ask
about thyroid nor B12 and I don't know if that was found later.
There's also "hardening of the arteries" that can affect the brain.
anyway, some years later, we had a family gathering and she was sharp as a
tack. ( back to her old self, although older).  Some people can slip into
depression and eventually psychosis.

So this mayo blurb (below) tells me to rule out drugs, alcohol (and some
people are especially good at hiding alcohol use) - an ex-neighbor of mine
comes to mind. She had bottles hidden all over the home and I really don't
think her husband clued in. She'd wait until he left, then have some
delivered by taxi. Even when I was there, if she thought I was busy doing
something, she'd turn her back to me, bend over, grab a bottle, down some,
then put the bottle back and continue talking like nothing happened. I saw
it, numerous times.
Mayo also mentions thyroid.
And autoimmune disease.
I don't know all autimmune diseases, but apparently in Lupus, psychosis can
occur. A quebecer told us that his wife was diagnosed with psychosis and
was in hospital for months, but once she was diagnosed with Lupus and
treated for that, that cleared up and her psychosis wasn't a problem any
more.
http://www.mayoclinic.org/specialty-pharmacy/hepatitis.htm
Side Effects
Side effects from drug therapy include severe flulike symptoms from the
interferon and a temporary drop in hemoglobin (anemia), white cell or
platelet counts in your blood. Chronic side effects, which affect about
half of those receiving treatment with interferon-ribavirin, include
extreme fatigue, anxiety, irritability and depression. A small percentage
of people may experience psychosis or suicidal behavior.
Interferon is not recommended for people who have a history of major
depression. Those who have untreated thyroid disease, low blood cell counts
or autoimmune disease, or who drink alcohol or use drugs are also not good
candidates for this treatment.

http://www.postgradmed.com/issues/2001/07_01/dharmarajan.htm
Vitamin B12 deficiency should be considered in the initial workup of
patients with dementia and psychiatric disorders (20,21).
Cerebral
Dementia, depression, memory loss, psychosis, cerebrovascular disease
(through high serum homocysteine levels)

Print that up and if she is low, tell them you want the injections. Some
people continue later on sublingual (under the tongue). Extra B pills don't
do it, because sometimes there's an absorption problem in the gastro
system, so injection is cheap and painless and once topped up, she can be
rechecked every six months and more injections, if necessary or continue an
injection monthly.

If you can't find the cause, then as Steph thinks, maybe it is the melanoma
and it's just not visible at this time. Which would say to me the
interferon isn't working.

My cousin died from melanoma spring/summer 2004. It was in his lungs, liver
and brain.
Nobody ever mentioned that he had psychosis. When they saw the mets in his
brain, they gave him RT (radiation therapy).

Antipsychotic drugs are usually pretty hard to tolerate (side effects of
one or the other).
If it was my mother, and was satisfied, they'd ruled most of the above out,
I'd want to make sure she was not spending too much time alone ruminating
(worrying about things), on an antidepressant, eating as best as possible
and out there with people of her age (if possible, along with a relative to
watch over her) as often as possible, doing things she would enjoy and only
put in nusing home and/or antipsychotics if those tactics didn't help.

I don't want to give you false hope, but I think it's fair to consider some
other possible reasons first.
Let us know, ok?
Keep in touch, there's always a hug lurking around here.
( ( Suz ) ) )
J