Hello My Gf Recently Got Here Regular Check Up Where

hello my gf recently got here regular check up where she had an pap smare and a blood test done. After viewing the results they said they found something in here blood but they still passed her off as ok and fine then she did a second check up last thur with a urine sample now she got a call this morning saying that she has chlamydia.. My question is they are trying to say it was from the first test that she had it is tht even possible when they were the ones who said all was well, or can this just be a mishap in the medical field overall?