Sti Advice pls

I recently started dating someone and after we had sex I started showing symptoms of chlamydia a week or two after. So I went and got checked and it came back positive. I told him about it and he’s saying there’s no way it could’ve been him. But I’ve only had protected sex with one other guy and that was half a year ago. Why would I just now be showing symptoms after sleeping with the guy I’m currently dating? I feel like he’s lying but I didn’t want to say that.. or he’s going to blame it on me.

And YES I have learned my lesson about being careless and not using condoms.