Just be glad you caught it early and it didn't cause any long term damage. Learn a lesson from it. In future-discuss sexual history and both get tested together before starting a sexual relationship.
It only takes one time to catch an STD. Why would she lie? Maybe an ex cheated on her or maybe he gave it to her at the beginning.. it is true that sometimes STD's have no symptoms and chlamydia is actually very dangerous to a woman if it goes undetected long term. She could be facing infertility.
I think you should believe her when she says she has only been with 2 people as it is possible her ex gave it to her
"Don't ask a question if you can't handle the answer".