Her family believe the move was intended to put pressure on Germany, one of the signatories to the nuclear accord, as Iran pushes for the lifting of US sanctions at talks in the Austrian capital, Vienna.
Germany backed the 2015 deal with France, the UK, Russia, China and the US. Former US president Donald Trump pulled out of the pact three years later.
Ms Taghavi told her family there was no indication that she was about to be moved after 20 days in a women’s wing. She had been expecting to go for blood tests for her diabetes, her daughter Mariam Claren said.
Ms Claren spoke to officials at Germany’s Foreign Ministry on Tuesday to urge them to highlight her mother’s case.
"I told them very clearly: if my mum is going to be leverage for the [nuclear deal] then that's really, really cruel. I want my government to stop this," she said.
“These talks in Vienna are nothing to do with my mum. I don’t want there to be a link but, to be honest, there is.”
Ms Taghavi was arrested in October last year during a crackdown on women’s and labour rights campaigners.
Her detention bears the hallmarks of other arrests of western citizens who were accused of crimes on spurious grounds and used as bargaining chips to secure the release of Iranian prisoners abroad or to pursue Tehran's political ambitions.
Last month, Ms Taghavi spoke to her daughter for the first time after five months in the IRGC part of the prison. She told Ms Claren she was questioned intensively about her beliefs.
She was able to speak to her family on Tuesday by telephone from Evin prison and told them her interrogator did not know why she was back in solitary confinement.
But he told her she could expect to be there for two weeks. Ms Taghavi has not been charged with any offence and has been given no details of any allegation against her.
Shiva Mahbobi, of the Campaign to Free Political Prisoners in Iran, said Ms Taghavi’s move comes as Iran faces pressure on a number of diplomatic fronts, including the nuclear talks.
“Until the negotiations get somewhere, they are going to move the dual citizens around. It’s quite predictable,” she said.
The EU is preparing to target eight Iranian militia, police commanders and state entities by imposing travel bans and freezing assets this week over the deadly November 2019 crackdown on protesters in Tehran.
The penalties will be the first imposed by the bloc on Iran for human rights abuses since 2013.
Individuals who will face sanctions include members of the hardline Basij militia, which is under the command of the IRGC, the most powerful and heavily armed security force in Iran.