Lab workers and physicians regard the IgM as a test for recent infection, and the IgG as a past infection.
However, The Herpes IgM is regarded as a flawed test. If it was 5 months post-exposure and you had anything, you should have developed an IgG antibody by now. Dr. HHH in the experts forum says a 12 week IgG test is accurate.
Grace who is the community leader on these boards, has a journal post about herpes blood tests you should read:
http://www.medhelp.org/user_journals/show/12667/herpes-blood-tests
From her post:
"...the herpes igm blood test ( what providers often tell you shows a new infection vs an old infection ) is not an accurate blood test for diagnosing herpes with. It should not be used in adults. You can search here on medhelp for zillions of posts by Dr Handsfield, Dr Hook and myself on it."
It is my opinion that no further testing is necessary. Congratulations on your negative IgG result!