IgM becomes detectable a few days after the infection. The presence of IgM antibodies
indicates an acute infection, and will be detectable for approximately 8 weeks or may be 10 weeks.(Rubella IgG
will appear almost as early as the IgM, but then persists for life).
It looks unlikely that rubella IgM is still positive after 5 months of miscarriage
Because rubella incidence is low, a high proportion of IgM-positive tests will likely be false positive. False-positive serum rubella IgM tests may occur due to the presence of rheumatoid factors or cross-reacting IgM, or infection with other viruses.
You must get it repeated again at some different laboratory .
Once you find that IgM is negative,which it should be,you may plan your next pregnancy