Chlamydia is a common STI that usually does not show any symptoms.
Like you have pointed out it is possible the chlamydia is a new infection that was passed by someone else, but there are also other ways to explain the positive result.
Its possible your gf had chlamydia from a previous partner and the test 3-4 months ago, did not pick it up. We call that a false negative result.
It is not common, but also possible to get a false positive result from a chlamydia test. The test shows positive but you do not have chlamydia.
It is also possible that you had chlamydia from a previous partner. Chlamydia is common and usually does not show symptoms, I have seen it hang around for a few years. It is common for us to see chlamydia sneak into new relationships this way.
I would recommend getting a chlamydia test and treatment. You just have to make sure you wait 1 week after treatment before having sex and that both of you have been treated. As chlamydia usually shows no symptoms it is common for us to see it passed back and forth between a couple when people were not treated at the same time.
Please leave a comment to let us know if this answers your question or if you need more information.
For other readers, please feel free to leave a comment, or let us know if this was helpful.