Hi! I wondered that myself and found that the info is a little hard to come by - I honestly think that no one wants to scare away breastfeeding mothers so they don't talk much about hormones. What I did find though indicated that breastfeeding raises prolactin which in turn suppresses estrogen, progesterone, and testosterone in women. The most common "side effect" from the lowering of these hormones (especially estrogen and testosterone) would be lowered sex drive, but is also linked to other post-menopausal symptoms like fatigue and depression. The depression might be sometimes offset by the oxytocin produced while breastfeeding, which can cause elevated mood. Once ovulation returns to the lactating woman, the hormones go back into their normal up and down spikes, but this in turn can affect milk supply.
Of course many websites are quick to point out that breastfeeding moms might be tired and have low sex drive simply because they are sleep deprived new moms or have emotional or nutritional issues. I will say though that I knew exactly when I first ovulated (after more than a year of breastfeeding) just based on sex drive and some physical differences. I think it just effects some women more than others. None of this info made me want to stop breastfeeding, but I found it useful to know!
Here are some links:
Columbia U Q&A page
Hormone changes during menopause
Pubmed article
