Chlamydia can lie dormant in the body a long time, either in you or in your husband. It is not necessarily proof of a recent affair. Have either of you been tested for it since your marriage before now?
The std aside.....often when people get caught cheating, they will try to minimize what happened to avoid the consequences of their actions. An emotional affair? Why did that happen? Was he actually sleeping with her but said that to save his marriage? Only you know the answers, and all the factors that led to the situation.
I agree with Annie.....you should check with the doctor to see if chlamydia was part of those tests.
So, here is apparently where you are.
You have been tested for chlamydia just recently, and have learned that you have it. You had a test around three years ago that you are pretty sure indicated you did not have chlamydia. (Please be entirely sure about this. We asked if you knew that you were tested specifically for chlamydia back then, and you said things like you had a swab or had a culture, as though a swab or culture can only be used to test for chlamydia. Swabs and cultures can be done for a lot of things. So the important question is, when you found out this past week that you had chlamydia, did you specifically ask the doctor at that time about the test from back when, and he actually looked in your records and confirmed that you were tested for chlamydia in 2015?) Don't mistake this if you are going to get into a fight with your husband. You sure don't want to find out later that you were never tested for chlamydia in 2015, opening the door to the possibility that you had chlamydia before you got married. It would be bad to be thinking of a divorce over a misunderstanding.
OK, then, let's assume you determine FOR SURE that you had a test specifically for chlamydia since your marriage and that it was negative. (I've been assuming all along that you have been with no other man since marriage.) This would mean you can have only gotten chlamydia from your husband. The question then becomes, could his chlamydia have pre-dated your marriage? The doctor's nurse said she had a feeling it was recent, but she might not be the best person to ask. I would sit down with the doctor and talk it over. Again, it would be bad to divorce over something that had been there before the marriage.
If the doctor can assure you that chlamydia would not have been your husband's body before marriage, and that you yourself didn't have it until after 2015 because the earlier test proved you didn't, then in your shoes I would take this all as total proof your husband has been lying. You already have the sense that he was, the doctor's information would just confirm it. If he is lying to you now, it's going to be because he is trying to preserve the status quo. But unfortunately, catching him lying is the beginning of realizing you will never be able to trust him. At that point it would be time to find a job (if you don't have one) and daycare, find a place to live, and see a lawyer. I'm so sorry. But he does not sound like he is innocent.