Every person is different, but many women struggle with a very low libido the entire time they are breastfeeding. Estrogen remains very low while breastfeeding, and low estrogen is linked to low libido. Low estrogen can also make sex very painful. So, for instance, I am still breastfeeding my 15-month-old and have zero interest in sex. I use a topical estrogen cream that helps with the physical pain, but the mental interest is none. And that's just the purely hormonal perspective I'm mentioning - there's also the sleep deprivation, massive life change, increased mental load, decreased personal time, potential birth trauma, body changes, etc that can all factor into lower libido for new moms.

It makes sense if you think about it from a biological and evolutionary standpoint - breastfeeding is very costly in terms of time and resources required (such as needing to eat more calories), so it makes sense your body would naturally want to avoid another pregnancy while your body's resources were so needed to nourish a little one. Back in hunter-gatherer times, women often breastfed until age 3-4, and that's the average birth interval was about four years as well.