I'm still breast-feeding my third baby, and my libido is completely gone. I don't even think about sex. My ob-gyn seemed to think it's related to breast-feeding. That was months ago, and I still feel the same. I feel bad for my husband. I know he is starting to take it personally.
I have to admit I've been letting your question sit here in my "good question!" file for months. As a newish mother myself, I can't easily write about this without taking it a bit personally too. I generally try to avoid getting all me-me-me unless it's particularly amusing, but sometimes it can't be helped.
Of course it seems related to breast-feeding. It is deeply and inextricably connected to breast-feeding, a process involving sex- and sexuality-related hormones, intimate touch, and boobs. So really, how could it not affect your sex life? Not to say that postpartum libido issues (I hesitate to call them problems since they are so natural, normal, and expected if generally unwelcome) are purely hormonal. You may be a big bag of hormones, but you're a specific, unique sack of hormones living a unique and specific life. You have a husband, and you have rather a lot of children. There's a lot more going on than the mere release or reception of this molecule or that.
I've read a ton on this subject, if not before I had these kids, then certainly since. And while most of what you see out there is common sense or nonsense, there are a few bits and bobs you may not have heard. Not everyone knows, for instance, that we all release the milk-making hormone prolactin after orgasm, producing a sensation of satiety. Even fewer people will have heard about the researchers who recently measured prolactin levels in laboratory subjects who masturbated to orgasm or had penis-vagina sex to the same end. The screwers released 400 percent more prolactin than the wankers did, possibly explaining why most people find partnered sex more satisfying than masturbation. Much work remains to be done (what about other kinds of sex with a partner? What about homo sex?), but if we in the Lactation Nation are already walking around with high levels of prolactin, which of course we are, we may already be feeling the sort of satiety that other, less milky people have to have partnered sex in order to achieve. We don't want sex because we feel like we just had some, and the drive to go get some more is suppressed. That's one theory, anyway.
Breast-feeding also releases oxytocin, that busy hormone with jobs ranging from stimuutf8g uterine contractions and causing your husband to start snoring so soon after sex to making prairie voles (and perhaps everyone else) bond to a partner and stick around to raise the children together. The oxytocin released at orgasm is responsible for the aaaaahhhh feeling you get as you nestle back into your beloved's arms. It creates similar warm fuzzies at the mere touch of the right person (good hugs release oxytocin, while unwanted or merely social hugs do not). Oxytocin, of course, is released as your baby nurses, but also by just cuddling with her (or in some cases thinking about her). Again, the sensations of calm, happy, shmoopy-pie satiety, while delightful, are not exactly conducive to going out and gettin' you some.
Add to all this the fact that your usual sex drivers, estrogen and testosterone, are at an all-time low, and the chemical basis of the "just don't wanna" that can last as long as you keep up the nursing becomes obvious. Add to that the sleep deprivation, the ambivalent (to put it mildly, also inaccurately) feelings that many of us harbor about the changes our bodies have gone through, and the vaginal dryness, and there ya go. Death of sex. For a while.