This is from the IgeneX website:
IgM Western Blot
"This test is a very sensitive indicator of exposure to B. burgdorferi. It may be positive as early as one week after a tick bite, and will usually remain positive for six to eight weeks after initial exposure. A positive IgM result with clinical history can indicate early Lyme disease, or even persistent infection in otherwise serologically negative individuals"
This is from the ILADS website:
"It must also be kept in mind that these antibody tests are not static but in fact change over time. Thus, a patient negative by the Western blot may seroconvert to a positive blot with treatment. Conversely, a patient positive for IgG response may develop another IgM response, suggestive of a recurrent infection. "
If the IGM becomes positive, it could be due to the immune system recognizing the antigen again. If the organism was evading the immune system, there would be no IgM. When the immune system "sees" the bug again, after it comes out of it's "hiding" places, the body may make IgM and it will appear as a new infection, when, in reality, it is just a recurrence of the old infection.