so confused

So about two weeks ago I went to the doctor and had a physical along with some tests. They tested me for chlamydia and it came back positive. I told my boyfriend and he freaked out on me thinking I’ve cheated. Which I haven’t but he had a few weeks before. So I told him he needs to get tested and he needs to tell the girl he hooked up with. He got tested a few days ago and he told me it came back negative so he didn’t have it. But he hadn’t told this girl yet. So I told her about an hour ago and she said she went in last week and she also didn’t have it. I’m confused on how they don’t have it but I do? I haven’t cheated on him or done anything with another person since we’ve started talking/ dating. Could I of had it before I met him? Is he lying about not having it? It doesn’t make sense to me that I would have it but he doesn’t. Any advice?