"I just was diagnosed 2 days ago with a rare form of Lupus called 'Lupus Myelitis,' she wrote. "I'm not on my death bead...far from it.I'm gonna be fine."
The 46-year-old actress said said on Tuesday that she had been "been really, really sick since early Sept."
"Full body muscle weakness to the extent that just walking difficult, stairs impossible and I even need a neck brace to hold my head up," she said, adding that she didn't want to name the disease.
"Please, allow me this time before I have to champion another issue," she said.
But, in apparent frustration with the media attention, Johnston named the disease.
"Oh for f***s sake. I'm not trying to be 'coy,' 'relevant' or 'get attention,' she wrote. "I just wanted to have a few weeks of privacy to navigate it & receive treatment."
Lupus myelitis is caused by an inflammation of the spinal cord. Symptoms include weakness in the limbs, numbness and back pain.
Johnston also thanked her colleagues at TV Land for their support.
I gotta say, TV Land and everyone at 'the exes' have been just amazing, all of them…so patient, understanding, loving & supportive. It's true that when you're sick, you find out who your real friends are, and these people, along with so many others, like.....Oh Crap. This is starting to sound like a dreadful acceptance speech, so I'll just say you know who you are. Thank you & I love you.
Celebrity Couples of 2014 [PHOTOS]