I have a tentative diagnosis of polymyositis. We just discovered mold in the ceiling above our bed. Can mold contribute to this type of illness?

Unclear. Polymyositis a disease which causes weakness in the muscles usually through inflammatory processes. Unfortunately it is unclear what triggers this process to start. Some viruses have been implicated but the majority is usually due to an autoimmune process. An autoimmune process is where your immune system starts attacking healthy cells, in this case the muscles.
Etiology. The etiology of polymyositis is believed to be autoimmune. If mold started a hypersensitivity type of reaction that behaves like myositis is possible even though it will be uncommon. Hope it helps.