Teresa Romero, 44, had been treating Fr Manuel Garcia Viejo, who became ill while working as a missionary in Sierra Leone.
He was flown home to Spain but later died.
It is understood that the nurse may have touched her face before taking off the gloves she had worn while caring for the priest.
Initial results suggest Mrs Romero is no longer infected but a second test is needed before she will be declared free of the illness.
More than 4,500 people have been killed by Ebola as the disease spreads across West Africa.
Romero was the first person to contract the virus outside of the area.
It is reported that she may have been given antibodies from Ebola survivors while being treated at a Madrid hospital, raising hopes of a potential cure.