How should I tell him?

My boyfriend and I have an open relationship and he likes for me to send him pictures of videos of me having sex with other men. So when I sent him the last one he said that I should get tested and said something about the guy possibly having something so I did. I just went to get my test results and everything else was negative except for Chlamydia. I’m glad it can be treated but the problem is that I don’t know who gave it to me.