A positive result can occur in anyone who has ever had a chlamydial
infection; it does not diagnose a current, active
infection. Further, some positive results come from past chlamydia-like infections of the respiratory tract, like bronchitis.
If you have been at risk recently for a new chlamydial infection (unprotected sex with a new partner and so on), then having a urine chlamydia test makes sense. But not because of your current symptoms or past prostatitis.