See how Sally Massagee and her doctors tried to stop muscles from growing all over her body on an "AC360°" special investigation, tonight 10 p.m. ET

(CNN) -- As a wife and mother to five children, Sally Massagee had always led an active lifestyle. She ran her own successful CPA firm in her hometown of Hendersonville, North Carolina, and loved to cook and play tennis with friends.

But in her late 40s, that all changed. Massagee's muscles began growing out of control.

"Everyone assumed that I spent a whole lot of time in the gym," she says. "One doctor said how many hours a day do you spend in the gym?"

But Massagee didn't work out at the gym, nor was she taking steroids. As her muscles continued to grow, they also started to harden like rocks underneath her skin, causing excruciating pain. Long gone were the days of running and playing tennis. Even basic activities like walking and standing soon became unbearable.

"I couldn't reach up to fold over the turtleneck on my sweater," she says. "I couldn't reach my ears to put on earrings."

In 2006, with her family's support, Massagee began to search for help. She headed first to Duke University Medical Center. For two years, she saw doctor after doctor and had test after test, but no one at Duke could figure out what was happening to her body.

For Massagee's husband, Buddy, the mystery was bewildering. "Duke's about as high as it gets on the notches in the Southeast," he says. "And when Duke tells you they don't know what it is and they don't have any more ways or ideas for guessing or testing what it is, it's a scary thing."

After Duke came up empty, Massagee applied to the the Mayo Clinic in Rochester, Minnesota. But they rejected her because she did not have a diagnosis.

With her physical capabilities slipping away, Massagee, 53, thought there was nowhere else to turn. "I felt certain that if there wasn't a diagnosis, I felt that it was pretty certain that it would kill me," she says.

In 2008, she got a glimmer of hope. She was told about a new program at the National Institutes of Health that takes on the rarest of undiagnosed cases. With a referral from her endocrinologist from Duke, she was accepted into the Undiagnosed Diseases Program.

"There just aren't words for how excited I was," Massagee says, "that there was a hope ... that someone would be able to find out what was wrong. And I remember Buddy saying in the process, if anybody can figure it out, I think these folks can; I think if there's any hope, this is it."

After the work at UDP, Massagee had a long and nerve-wracking wait.

Finally, in May last year, she got the call she'd hoped for -- the mystery was solved.

In her case, abnormal proteins which normally attack the organs, were attacking her skeletal muscles.

Although the mystery was solved, Massagee's story was far from over.

Her diagnosis gave her hope that she'd survive. It also meant enduring a grueling new chapter of her life with no guarantees how that would end.

Watch Anderson Cooper 360° weeknights 8pm ET. For the latest from AC360° click here.