Hey guys I recently went to the OBgyn for my yearly pep

I have been with my boyfriend for over a year and a half. I got tested 6 months into us having sex all test came back negative... I then went and got my yearly pep which means I got tested again 2 weeks ago positive with Chlamydia.. My boyfriend is the only person I’ve had sex with in almost 2 years. Do you think he cheated on me?

Also how do I tell him? I’ve never had something like this and honestly I’m devastated