(CNN) -- Less than a month after two lung transplant surgeries to save Sarah Murnaghan's life, the 10-year-old has developed pneumonia in her second set of replacement lungs, her mother posted on Monday.
Sarah has developed pneumonia in her right lung, her mother, Janet Murnaghan posted on her Facebook page. Physicians believe it was caused by aspirations from her belly, she wrote, meaning contents from her stomach went through the wrong pipes and into her lungs.
"Yesterday was tough," Murnaghan wrote. "Today she is more stable, but this is definitely a large set back."
The lungs were infected with pneumonia before they were implanted, but this is a new infection.
Pneumonia is not uncommon after any type of lung surgery, CNN's Sanjay Gupta said.
"Sarah had an even higher increased risk because she's taking immune-suppressing drugs and because of her cystic fibrosis," Gupta said. "But even with a setback like this infection, nearly 80% of people survive at least one year after lung transplant."
Sarah was born with cystic fibrosis.
Her family pushed for a change national transplant policy as they fought for her to be eligible for adult lung donations. Her family had been told she had five weeks to live in May, and she had been on a transplant list for children's lungs for 18 months
After the Murnaghans petitioned, the Organ Procurement and Transplant Network's executive committee approved a one-year change that makes children younger than 12 eligible to be on adult lung transplant priority lists.
Sarah received her first transplant lungs June 12, one week after she became eligible for adult lungs. The donated lungs were poor quality, but the family proceeded with the surgery because "Sarah was out of time to wait."
The lungs failed shortly afterward, and Sarah underwent emergency surgery to put her on a bypass machine to keep oxygen pumping through her body.
Family members knew the lungs were infected with pneumonia before the surgery, but they went ahead with the transplant because "they were Sarah's best and only hope."
Despite the infection, her family had reported steady improvement up until Monday.
In the same post, Janet Murnaghan thanks everyone for their thoughts and prayers.
"We have an amazing team of doctors who go above and beyond but also walk this road with us in such a kind and compassionate way," she wrote. "Thankful to God for this day!"