Chlamydia. Why has it come back?

I had started going with my current girlfriend several months ago and went for an STI test - i had some of the symtons associated with Chlamydia - and both had Chalmydia. Got treated, didnt have sex for the week and the symtons were gone.
Since then we have been having regular unprotected sex.
We had anal sex (drunk) around a week ago and the symptons have returned.
Im going for a test in the next few days but im almost certain i have it again and she has been saying she has been feeling uncomfortable since it happened.

Why has it come back? Can chlamydia do this? I know i havent been with anyone else since being tested and im pretty sure she hasnt