(Photo Credit: Toby Canham/Getty Images)

Emmy-winner Kristen Johnston tried to keep her sickness private, but with the actress having taken a short leave of absence from her show "The Exes," she thought she needed to let fans know what was going on.

"I've 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 wrote Tuesday on her Facebook page. "Through the fall, I shot as much of The Exes as I possibly could, but eventually I did have to (BEGRUDGINGLY) miss two episodes."

Johnston, 46, said "Dancing With the Stars" alum Leah Remini would be filling in for her starting Jan. 1, adding, "I promise, you won't even miss me!"

RELATED: Kirstie Alley on Leah Remini - 'She's a Bigot'

"After 4 months & 17 Drs (no lie) I've FINALLY been diagnosed with a rare auto-immune disorder," she added. "The good news: I'm finally on the right treatment, feeling better & back at work!"

After the former "3rd Rock from the Sun" star purposely left out what autoimmune disorder she was suffering, she decided to let everyone know Wednesday that it is lupus.

"I just was diagnosed 2 days ago with a rare form of Lupus called "Lupus Myelitis" … I'm not on my death bead … far from it. I'm gonna be fine," she said. "The only reason I said anything is because I won't be in a few episodes."