I actually had the same experience. A week after having sex I went to get an STD test and my igm result came back positive. The doctor said that a positive igm means that I was recently exposed, but my body had no time to develop the igg antibodies yet. It totally freaked me out, and I had to wait two agonizing months before I could retest.
Well, two months later I went for a retest and I was negative! I really think the igm has a high rate of getting false negatives, or finding things that just aren't there. The point is, just wait the 3 months and don't worry about it. Pretend you never had an igm test because the results really can't be relied on.
As for spreading the virus without symptoms, that is a possibility. The herpes virus, after an outbreak, will hide in your nerve endings and be dormant, only reappearing when it is triggered by stress, illness or other factors. But people do "shed" the virus about 5-10% of the time out of the year (not sure of the exact number) even when there is no visible symptoms. So even if you are asymptomatic you can still spread the virus.
the herpes igm test is a greatly flawed test. your provider should get out of the habit of ordering it.
http://www.medhelp.org/posts/STDs/Confusiion-over-other-IgM-Herpes-posts/show/248394 is a prior post on why the test is flawed and why a + igm result for herpes isn't of much use. odds are your igm also wasn't type specific either which is even less useful in general.
grace