I have been tested for chlamydia via vaginal swab and also urine test, both of which were negative. Is it possible to be exposed to chlamydia and pass it on to someone else without contracting it yourself? And no, I did not have sex with two people in the same day.
It is quite possible to contract it w/out showing symptoms, but you cannot pass on any disease w/out first contracting that disease. If you contracted it, the tests should have been positive, regardless of whether you were showing symptoms. However, I don't personally know the accuracy of these tests.
edit: I shouldn't say that you absolutely CANNOT pass on a disease w/out contracting it first. Theoretically, one could retain another person's semen or blood in her vagina (w/out contracting the disease) and then pass it to a third person. But as you said that you didn't have sex with two people in one day, this can't be the case...