Got an std.

So a few months ago my bf and I, had broken up for about a month. We got back together and a few weeks later found out I was pregnant.. when I went to the drs, they checked for stds and mine came back positive for chlamydia. I knew he had sex with one girl when we were broken up because before we got back together we talked about it, but he said he used protection which is highly unlikely because I came back positive for this, and I DID NOT sleep or even touch anyone else when we were broken up. I was working like a dog to even think about it! But here's my thing... it's been about 8 months since it happened and I'm not quite sure if I believe he got it from the girl he said he must've gotten it from. Would it be weird to ask her? Not rudely or anything, actually wouldn't even know how to bring that up or come about that but if I were, how would I do it? Mainly just want closure about it. But I do think that if he's telling the truth I told him he should tell her because maybe she doesn't know and she could spread it but he didn't want to. He said it would be awkward to say, red flag to me. 
How do I go about this, or should I just drop it because it's in the past and I am choosing to be with him?