Sarah Murnaghan, the 10-year-old Irish American girl who received adult donor lungs last month now has pneumonia in her right lung, her mother told the press on Monday.

According to Sarah's mother Janet said that doctors believe the pneumonia was caused by 'aspirations from her belly,' or breathing stomach contents. She added that Sarah had a 'hard day' on Sunday but was more stable on Monday.

Sarah reportedly appeared to be responding to antibiotics but still required breathing support from a ventilator, her mother said. Janet called the unexpected illness as a 'large setback.'

Sarah received portions of a set of adult lungs on June 12 but those failed to work properly and she received another transplant of adult lungs on June 15.

The second set of lungs had pneumonia but the infected area was removed before the transplant, Janet Murnaghan said. A further operation performed last week to improve the function of Sarah's diaphragm was a success.

Murnaghan's parents successfully fought a rule preventing her from qualifying for adult lungs. Before the surgeries Janet Murnaghan called attention to the so-called Under 12 Rule, which said that even though Sarah would be given priority when pediatric lungs became available, adult lungs would have to be offered to adult matches in her region before they could be offered to her.

On June 5, a federal judge issued a temporary restraining order to prevent U.S. Health and Human Services Secretary Kathleen Sebelius from enforcing the rule for Sarah.

By June 10, the Organ Transplantation and Procurement Network re-evaluated the Under 12 Rule and decided to keep it but created a mechanism for exceptions, permitting the surgery to go ahead.

Sarah was dying of cystic fibrosis this time last month, but now is responding to treatment after her family fought for a change in lung-allocation rules.

10-year-old Sarah