I'm so confused

yaya
Alright I went to the gyno and I have chlamyida but what I don't understand is how my boyfriend was my first and he never cheated and he got tested he doesn't have it so how did I get these std if he never had it 
UPDATE: ok so he had chlamyida before we started to have sex but he got treated so how I get this if he got treated that doesn't make sense