I have a very confusing situation. My boyfriend and I have been together over a year. We both had partners before each other, but have always used condoms. Since we have been in this relationship for a year, and have both been tested for STIs, and I am on birth control, we have started to use condoms less. We use them when we remember but if its a spur the moment kind of thing, we don't worry about it.

Recently, my boyfriend approached me and said he has an STI (chlamydia) and I should get tested. I was mortified! I had just been tested two weeks before and everything came back negative. His previous testing had also been all negatives. I went back and was re-tested, and again the results came up negative (I was given a prescription just in case).

Neither of us have ever had sex without a condom before now. How is it possible that he suddenly developed an STI when both of us have tested negative before, and haven't been with anyone else? Could it be a misdiagnoses, or is it possible to get an STI another way? For example, having unprotected sex while the girl is on her period? Help!!

Here are the possibilities, in order of most likely to least:

• He WAS with someone else over this last year. I know that's certainly the least easy possibility to look at, but if he really tested all clear before the last round of tests within the amount of time you two have been together, and you also have had fully negative tests in that time -- and are testing clear for Chlamydia now -- then it is a strong possibility you need to deal with and address, especially if you're still having sex with this person. I know it's not something anyone wants to consider, but people who agree to be monogamous do, as a group, break those promises pretty regularly.

• When he said he had been tested before, he was either dishonest with you, or thought that testing was something he got in regular doctor's visits, and didn't know any better then. So few straight men -- compared to women: it's about one man to every eleven women -- get STI testing at all or regularly, especially younger men. So, many have no idea what it entails and that it's usually something you have to ask for: we get guys every day who just assume that when you get regular checkups with any kind of urine or bloodwork, if they have an STI, a lab technician will just happen to see it, not understanding that lab tests just don't work that way: technicians find (or do not) things they are specifically looking for. testing isn't random or happenstance. If that was the case, he may well have had Chlamydia since his last partner. And in that is also the possibility that he wasn't being honest about using condoms all the time with previous partners, but too, safer sex is exactly that: safer, not 100% safe. Condoms do a great job of reducing the spread of disease, but again, they reduce it, they don't eradicate it. So, it's also possible (though not likely) he did always use latex condoms, but got it anyway.

• He DID have Chlamydia previously, and somehow, his previous test just didn't catch it. Lab errors do happen sometimes. And given they can happen in any number of ways, he could certainly go re-test himself to verify that his positive now for Chlamydia was correct.

You having your period and having unprotected sex has nothing to do with anything when it comes to this. Menses doesn't create Chlamydia, nor does it develop due to menses. To give someone Chlamydia, you'd have to have it yourself, and then spread it to someone else via vaginal, anal or oral sex (though it's far less likely to be transmitted orally: it is primarily transmitted though genital-to-genital sex).

Understand, too, that the incubation window for Chlamydia is pretty small: just one to two weeks. I don't know when your boyfriend was last tested (or why he just retested now?), but if he had a clear result for Chlamydia at his last test, and was not sexually active with anyone with the infection just prior to the tests, then it's relatively safe to say he acquired it since then.

No matter what, since an STI did crop up into the mix, you'll both need to take it from the top with safer sex practices again.

That means that for another six months, at least, you need to use latex barriers again, always, stay monogamous, and each have another set of tests at the end of that time period before you consider going without condoms again. And to be sure you have it all down for the future, before going without condoms with any partner, you want at least TWO full and negative screenings, one ending with that six month period of latex barrier use.

Talking about these possibilities isn't likely to be easy, but I strongly suggest you do discuss them, however hard it may be. Particularly if your partner isn't really being monogamous, that's something you need to know, both for the sake of your health, and also so that you can be sure that you're in a relationship you still want to be in, particularly if whatever rules you've both made aren't being followed by one of you. Whether your boyfriend cheated or not, this conversation is likely to be very intense and heated, so I'd suggest you be as calm about it as you can be, makming clear that honesty is the order of the day, and whatever the truth is -- even if it's one of the tougher ones -- it's important you both know the real scoop. You may even just want to print this answer out to share the possibilities so you both can get to the bottom of this and figure out what exactly it is you need to deal with here, and what you don't.