Isn't there a thing where you can get tested for genital chlamydia and miss oral chlamydia, and vice versa? That might explain why one partner would test positive and the other would test negative, either then or now.
So I'm not a doctor - we don't have any doctors on this site.
But it seems to me either one of two things could have happened. Keep in mind I don't know anyone involved, and these are just based on facts about chlamydia.
1 - She got tested and treated before you did, and then when you tested, she got another test that was negative. The timeline on that would be really tricky, because it would count on you not having unprotected sex with her during and after her treatment until you got treated. It's possible, though.
2 - You got a false positive test result, and never had chlamydia. This is unlikely because you had symptoms.
How did you get chlamydia? If you get chlamydia symptoms, it usually happens shortly after infection, so it would have been a new infection, not an existing one from prior to when you and your girlfriend met.
I'm wondering if maybe you're missing what your partner was lying about. I don't know, though - I don't know her. Something isn't adding up here.