How could you have chlamydia if your partner does not

Very unusual: Chlamydia is a sexually transmitted disease. Therefore, if one has it and your partner does not, the diagnosis must be questioned or you must have several other sexual partners where it was contracted.

Got it somewhere: You had to get it somewhere. Either your current partner or a prior partner. Sometimes it doesn't have symptoms and you can pass it on without knowing. Best to treat both partners.