STI’s

I went to the doctor a few days ago to get tested for a UTI and get my regular exam, and I found out that I have chlamydia. I don’t even know how this is possible. Me and my partner have been together for a month, and with his schedule I highly doubt he’s been with anyone else and I KNOW I haven’t. Either way, I have to tell him. But I’m worried he’s going to think I’ve been cheating. How do I tell him without him accusing me of something? Could it be possible that I got it from someone else a long time ago and just noticed? Or maybe he did? Please help I’m freaking out.