Imogen D'Arcy, 13, also spent hours on the Internet to find out how to hide her suicidal thoughts. Eventually she hanged herself.
The tragic girl's family said they were not aware of her having an eating disorder.
But a note found at the family's home in Leeds by her father Paul revealed his daughter, who played football and enjoyed running, had issues with her weight.
'I took it to the bedroom and read the first paragraph but I couldn't read any more,' he told Leeds Coroner's Court.
'It appeared Imogen had a problem with her body image, she felt fat and ugly. This is not true she doesn't have an ounce of fat on her.'
The family also found out that their daughter, who was head girl material at Bradford Christian School, West Yorkshire had been accessing anorexic and suicide sites.
The court heard on the day of her death, on December 1 last year, the shy teenager, affectionately named Immy by her family, had been for a haircut with her mother.
But the pair had argued after Imogen refused to tidy up her sibling's toys.
When the family had later gone shopping Imogen, who played the clarinet and flute, was left on her own with her then two-year-old sister, Rachel.
But when they returned to the house Mr D'Arcy found his youngest daughter watching TV and asked her where Imogen was before the toddler replied 'Immy is lost'.
He then discovered his daughter in the family bathroom and administered CPR.
A text message she had sent from her phone read: 'My mum swore at me and I couldn't take any more.'
Imogen was taken to Leeds General Infirmary and later to the Martin House Hospice where her life support machine was switched off eight days later.
DI Scott Wood, of West Yorkshire Police, told the inquest: 'There is no doubt she tried to take her own life.
'We later found on her computer the lyrics to a song called Courage by Superchick. The lyrics seem to be about personal suffering from anorexia.'
Coroner Melanie Williamson passed a verdict of suicide, Daily Mail reported.
She said: 'Imogen was a fit and healthy 13-year-old girl, a popular young lady and fitted in well with her colleagues.
'She was a perfectionist and like so many others she was affected by her shape, weight and size.'
After the inquest, the girl's mother Susan exhorted other parents to monitor netsurfing by their children.
'We would urge parents to exercise extreme vigilance in overseeing the use of the internet by young teenagers.
'Imogen had no history of depression, to a certain extent she built the fantasy around her. In the end we think it was teenage angst.
'Imogen was a shy, gentle and kind-hearted girl. She was performing well academically at school and was well liked by her friends and teachers.
'That Imogen would choose to end her life in such a violent manner was completely unforseen by everyone who knew her.
'The pain and anguish of losing our daughter through suicide is indescribable,' Susan said.