I didn't catch chlamydia but my girlfriend did

[Orhasm]

So my girlfriend and I have been having sex for a little bit longer than 2 months and we got tested around a month in. She came up positive and I came up negative. I want to say she's cheating on me but I planned out all the time we've been together and we were together, having continuous amounts of sex and I just don't see where she would've had the time to cheat.

She says that she got it from someone before me back at the end May of 2017 but that means she would've had it for two months without me me getting it. Is there a way that I'm more immune to chlymydia or does she have to cheat in such a short amount of time for me to get it?

Thanks to all those who respond.