Silva Kashif was punished without her family being told after she was arrested while walking alone near her home in the Sudanese capital, Khartoum.
Her mother, Jenty Doro, said: ‘I only heard about it after she was lashed. Later we all sat and cried.
‘She is just a young girl but the policeman pulled her along like she was a criminal. It was wrong.
She said she would sue the police and the judge because her daughter is a Christian and underage.
The law states that under-18s should not be given lashes.
Doro said Khashif was taken to Kalatla court where she was convicted and punished by a female police officer in front of the judge.
'I only heard about it after she was lashed. Later we all sat and cried ... People have different religions and that should be taken into account' she said.
Khartoum is governed by Islamic sharia law. But although Miss Kashif is living there she is originally from the south of the country, which is not.
The government is supposed to be working to soften the impact of sharia for southerners living in Khartoum.
Her lawyer, Azhari al-Haj, said: ‘She was wearing a normal skirt and blouse, worn by thousands of girls. They didn’t contact a guardian and punished her on
The case will add fuel to a debate already raging over Sudan's decency laws after this year's high-profile conviction of Sudanese U.N. official Lubna Hussein, who was briefly jailed for wearing trousers in public.
Hussein, a former journalist who used her case to campaign against Sudan's public order and decency regulations, is touring France to publicise her book about the prosecution.
She had faced the maximum penalty of 40 lashes but was given a lighter sentence.
Arrests for indecency, drunkenness and other public order offences are not uncommon in Khartoum which is governed by Islamic sharia law.
Earlier this year Sudanese UN official Lubna Hussein was briefly jailed for wearing trousers in public.