Dr Marie Anne Vandenhende, 33, told the High Court in Glasgow that security guard Clive Carter had been holding a fire extinguisher and told her not to use the women's toilets because of a fire.
But after using another convenience, she and a friend decided to return and investigate.
The jury heard Dr Vandenhende saw Khanikporn Satjawat lying in the cubicle covered in blood and that "what was left" of the left side of her face was badly injured.
Dr Vandenhende was giving evidence at the trial of Mr Carter, 35. He is accused of murdering Miss Satjawat by hitting her on the head and body with a fire extinguisher at the Clyde Auditorium on November 12 last year.
He has admitted culpable homicide and lodged a special defence of diminished responsibility, but the Crown has not accepted his plea. Mr Carter, from Motherwell, North Lanarkshire, is also accused of committing a breach of the peace at a hotel the week before.
Dr Vandenhende told the court she was at a medical conference at the SECC on November 12 last year, and went to a lecture with her friend, Sabrina Caldato.
They went to the toilet around 2pm and the door was opened by Mr Carter. The witness said he was holding a fire extinguisher and told them there was a fire.
Advocate depute John Scullion, prosecuting, asked: "How did he seem at that point?" Dr Vandenhende said he looked agitated. "His face was a bit red," she said, adding: "He was talking really fast."
Mr Scullion asked if she thought there was a fire in the toilets and said "absolutely not".
The witness said she and her friend went to find other toilets but afterwards went back to where they had seen Mr Carter.
Her friend had spotted blood on the security guard, and, she said: "The whole situation was a bit strange"
The court heard Miss Caldato went into the toilet first, and Dr Vandenhende was behind her.
Mr Scullion asked: "When entering the toilet, what did you see?" Dr Vandenhende answered: "Initially I didn't see anything. Sabrina looked to the left then to the right and then she turned round towards me and she looked really pale and she said 'I'm not joking, there's a body.'"
Dr Vandenhende told the jury there was a body in a cubicle. Asked what she saw she said: "A lady lying down with lots of blood round her head and her body.
"I went to see inside the cubicle to see if the person was injured, to see how the person was. She had blood all around her head and on her body and she was not moving. She didn't seem to be breathing."
Dr Vandenhende said the two women left and Miss Caldato asked someone to call police. Dr Vandenhende went back to check if the woman was "really dead".
Mr Scullion asked: "Did you examine her?" She said: "I didn't touch her because … I was scared to get my prints, or remove some evidence and I was a bit shocked, and all I could think about was all the crime series on TV telling you not to touch anything."
Dr Vandenhende said the woman had "a very serious injury on the left hand side of her face".
She was asked: "When you looked at her did you see the left side of her face?" The witness said: "What was left of it." She told the court that it was clear to a doctor that the woman was dead.
Mr Carter denies the charges and the trial before judge Lord Matthews continues.