Writing on Instagram, the actor recalled how she recently tripped while walking down the street, resulting in an ankle sprain.
“I don’t remember it happening,” she wrote. “Or the pain. I was told. And my ankle is sprained and it makes me feel even more fragile. A simple ankle sprain.”
Blair also revealed she is suffering from face and neck spasms and recently caught stomach flu from her eight-year-old son, Arthur, whom she had with ex-partner Jason Bleick.
The 47-year-old said she felt concerned for her “future as a single mom”.
“I’m not dying any more than anyone [else],” Blair continued.
“I am just hurting. It feels like I am just breaking down. So there’s a truth to give to anyone else feeling this way. It’s just miserable. And scary. To feel unwell. I am so sorry.”
Blair went on to say that she had written her post in the early hours of the morning and thanked her followers for supporting her.
“May the silver lining surround us all,” she wrote. “And guide us out of the darkest. Thank you all for being the biggest loves.
“I am in a slow time. I need to recover. I want to recover. And I don’t know what that even really means. I send you all a reassuring warmth. We all need it.”
Blair was diagnosed with MS in October 2018.
According to the NHS, MS is a condition that affects the brain and the spinal cord, sometimes resulting in serious disabilities such as problems with vision, arm or leg movement and balance.
Other symptoms can include fatigue, difficulty walking, muscle stiffness and numbness.
For more information on support for those suffering from MS, visit MS Society.