Is rheumatic heart disease an autoimmune disorder?

Rheumatic disease. In a sense, yes. You get a strep pharyngitis infection that creates millions of antibodies to fight the infection. Some of those antibodies can turn rogue and recognize a component of the heart valves as foreign, leading to an immunologic attack on the heart valves.
Yes. Rheumatic heart disease generally follows untreated strep throat or rheumatic fever (which is also caused by streptococcus) because the body produces antibodies against the streptococcus bacteria that also react with proteins in the heart, causing rheumatic heart disease.