Amna Dirar, secretary-general of the opposition Beja Congress, a political party with a military wing active in eastern Sudan, was arrested and released on Tuesday and was asked to return for questioning on Wednesday.
"They detained me and kept me there for five hours for questioning," Dirar said on Tuesday.
"I have to report back to state security headquarters tomorrow morning."
Dirar recently returned from neighbouring Eritrea where she met other Beja Congress representatives and discussed conditions for possible talks with the government.
State security forces asked her about the meetings there, she said.
"I think because I am a woman they don't want to arrest me immediately ... so they are keeping me coming and going instead," she said.
"They want to put pressure on our people so they come to negotiate with the government and reach an agreement."
Eastern Sudan is one of many outlying regions of Africa's largest country where residents complain of neglect at the expense of central Sudan around the capital, Khartoum, the homeland of most of the country's ruling elite.
Khartoum accuses Eritrea of helping rebels in eastern and western Sudan, charges Eritrea denies.
Rebels are active in Sudan's east, but on a much smaller scale than in the south or the western region of Darfur.