Fardos Al-Toum, age 19, was one of 10 young women students arrested outside a church in Khartoum on June 25 for wearing jeans and long shirts, according to the group's lawyer, Muhamad Mustafa.
When Al-Toum showed up to court to answer the charge, the judge said the clothing she was wearing then was also indecent and sentenced her to an additional fine, plus 20 lashes, the lawyer said.
Mustafa said he had lodged an appeal against the lashes, but it is still pending, with no scheduled decision at the moment.
Five of the women were ordered to pay fines. Two of them, including Al-Toum, had to pay 500 Sudanese pounds (about $82) and three were told to pay 50 pounds.
Four of them were cleared of the charge and immediately released. One other is due to stand trial on Sunday.
Rights group Amnesty International has started an online campaign in the United Kingdom asking people to add their name to a letter to the Sudanese authorities in support of Al-Toum. More than 40,000 people have already done so.
"Flogging and other forms of corporal punishment should never be used as punishment -- they constitute torture, and should not be inflicted as part of a justice system," the rights group said.
"Moreover, these women have committed no crime -- they have instead been subjected to random, vaguely worded, discriminatory laws."
According to Amnesty International UK, the women were accused of violating Article 152 of Sudan's 1991 Criminal Act, which forbids "indecent or immoral dress" and carries a punishment of up to 40 lashes and a fine.
Although the law can be applied equally to men or women, the rights group says, in practice "it is disproportionately women who are discriminated against" under it.
The women arrested outside the Evangelical Baptist Church in Khartoum North are aged from 17 and 23, and come from the Nuba Mountains in South Kordofan state, where many of Sudan's Christian minority live, Amnesty International said.
The vast majority in Sudan are Muslim, with Christians making up only about 3% of the population, the rights group said.
It's not the first time the treatment of Christians in Sudan has sparked international controversy.
Last year, Mariam Yehya Ibrahim was sentenced to death in Sudan for refusing to renounce her Christian faith, after she was found guilty of charges pf apostasy, or leaving Islam and converting to Christianity, and adultery.
She was subsequently cleared, freed
and allowed to travel to the United States
-- but not before she had given birth in prison