Girlfriend tested positive for chlamydia but i tested negative. only been with each other. how can that happen?

Not unusual: It's common to find chlamydia in only one person in a couple. You might still have it; the test misses 5-10% of infections, especially on urine. Chlamydia clears on its own over time, maybe happened before you were tested. It can be carried for months, so maybe she was infected before you were together. Or you had an antibiotic for something else. Or someone had another partner. You still need Rx.