Welcome to the forum. I'll try to help.
Your symptoms are not suggestive of herpes, and I would not have recommended testing for it. My main advice is that if you have not been professionally evaluated, i.e. if your testing so far is only through an online source or lab, your next step should be to see your primary care provider or your local health department STD clinic. If your symptoms are related to your oral sex exposure 2 weeks ago, nongonococcal urethritis (NGU) is much more likely than herpes, but it requires in-person evaluation for diagnosis.
You are correct that the HSV IgM antibody test "sometimes can be wrong", but that understates the situation. In most cases, the IgM result is completely unreliable. Here is a thread that explains the situation in detail; although 4 years old, the information remains accurate:
http://www.medhelp.org/posts/show/248394
As the other thread says, false positive IgM tests are not usually due to cross reaction with other herpes group viruses; your past chicken pox probably has nothing to do with it.
In response to your closing question, I see no need for additional HSV testing. However, I cannot guarantee you aren't incubating a new HSV infection, so you could consider a follow-up IgG test (don't repeat the IgM!) about 3 months after the sexual exposure. In the meantime, if your symptoms are continuing, see a health care provider.
Good luck-- HHH, MD