A deaf woman says staff at the hospital she was born in "interrogated" her because they did not believe she was English.
Dena Bryant said nurses at Grantham and District Hospital refused to believe she was born and raised in the town.
United Lincolnshire Hospitals Trust said it was investigating the incident and would talk to Ms Bryant.
It said it was unable to comment on individual cases.
From April, NHS hospitals in England will have a legal duty to charge overseas patients upfront for non-urgent care if they are not eligible for free treatment.
They already have a legal obligation to check whether people are eligible, or not.
The NHS website states: "If you are not ordinarily resident in the UK, you are considered to be an overseas visitor and may be charged for NHS hospital services."
However, Ms Bryant, who lives with her mother in the nearby village of Great Gonerby, and is of native American origin, said: "I am a born and bred Gingerbread girl (nickname for a person from Grantham), who was interrogated as to whether I was English or not."
She said one nurse told her: "You do not look English, and you are not white."
Despite repeatedly saying she was born in Grantham, Ms Bryant, who is profoundly deaf, and lip-reads, said staff did not seem to understand her.
"I was standing there, thinking about running away - I felt frightened," she said.
Ms Bryant, who attended the hospital with an arm injury, said she finally saw a doctor, who was the only person to acknowledge and note she was deaf.
Maxine Hughes, head of operations and clinical services at United Lincolnshire Hospitals Trust, said: "We are sorry to hear of Ms Bryant's concerns.
"We will of course investigate the complaint and talk to Ms Bryant as soon as possible."