Arlene Arkinson was 15 years old when she disappeared after attending a disco in Bundoran, County Donegal.
The witness, Patricia Quinn, had to be excused after just 10 minutes in the witness box.
It followed a series of angry responses to questions about lies she had admitted telling.
Arlene was last seen being driven away by convicted child killer Robert Howard. Her body has never been found.
Howard had been on bail, accused of rape, and was living at Patricia Quinn's home at the time of Arlene's disappearance.
Mrs Quinn is the mother of Donna Quinn, a friend of Arlene.
She told the inquest: "I'm under so much pressure the last 21 years.  I've been hassled, battered and kicked.  Why should I not be angry?"
She added:  "I'd love the cutty (Arlene) to be found, everyone needs a decent burial."
The inquest was then adjourned for a time to allow Mrs Quinn to calm down.
However, it was subsequently decided she should not give further evidence on Monday. It is expected she'll return to the witness box next week.
Later, Mrs Quinn's son Mark gave evidence after his mother had left the court.
He told the court he had been in Bundoran on the same night as Arlene's disappearance, but had been with a different group of people.
He added that Robert Howard was his mother's boyfriend at the time and said,
"I never liked the man, never liked him from day one. You just know with someone. Never liked him," he said.
"I did say get rid of that boy. She said 'if you don't like it you know what you can do'."
Mrs Quinn has insisted the relationship was platonic.
Later, Mark Quinn agreed with a barrister for the PSNI that his mother was probably "infatuated" with Robert Howard.
He also agreed it was nonsense to suggest that Howard was living at their home at the time of Arlene's disappearance simply because the police had put him there.
Mrs Quinn has testified that Robert Howard was an informer.
Mark Quinn told the court that Howard had been living between the Quinns' house and his own flat, and often stayed overnight with his mother.

About 30 people were stuck on the ride when it was stopped at about 12.00 BST.
A spokeswoman said a piece of rubber "came away from one of the carriages." "At no time were any guests at risk," she added.
The ride reopened in March, nine months after the horror crash.
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In a statement, the theme park said all visitors were evacuated from the ride in 30 minutes.
"The ride remained closed for a short period to investigate the matter, which is part of our standard process in assessing minor incidents," it added.
"The technical team have declared the ride safe and The Smiler re-opened at 16.22."
Liam Dennett, 33, from Cheltenham, was in the queue for the ride when an announcement was made about a "technical fault".
He said: "The ride's still closed now. We can see it in the distance that it's still closed. I would say between 30 and probably 35-40 minutes that they were stuck for."
He said he would not go on the ride after seeing what happened.
"My son still wants to go on it, but I wouldn't go on it personally now," he said.
Two women needed amputations after two carriages crashed on the track in June 2015.
Merlin Attractions Operations Ltd admitted breaching the Health and Safety Act after an investigation found the crash was caused by human error.
A witness at the inquest into the missing schoolgirl Arlene Arkinson has reacted angrily to cross-examination by a barrister for the Arkinson family.
[NEXT_CONCEPT]
The Smiler rollercoaster at Alton Towers had to be closed after debris fell from a carriage - 14 months after it crashed leaving five people seriously injured.