It is important to keep in mind that diagnostic tests are not 100% accurate. There are false-positive results with any tests, and the type of test can determine how sensitive it is. Here is a link to various tests with information and various tests for chlamydia:
Did she have a pap smear a few months ago that came back normal/negative? A pap is not a test for chlamydia. If she did have a specific test for chlamydia a few months ago, do you know why? And what kind of IUD did she have?
In some cases, it is impossible to determine who exposed who...and when.