Sorry, I was thinking of the incubation period. Testing window is much sooner, as auntijessi says.
It's not 21 days for chlamydia (or gonorrhea) - it's 5 days usually.
Was it a urine or swab test? If it was, your 12 day test was conclusive, and you don't have either.
I have heard it's more like 21 days (for chlamydia). Check with your doctor or a website like the Centers for Disease Control, the information should be readily available.