no personal experience of this, but it's partly genetic. The whole purpose of life, for our genes, is having babies to perpetuate the genes; once you've had one, you're sort of programmed to lose interest in your husband and concentrate on your baby, which will need years of care before he's old enough to pass the genes on further.
But it's also partly physical; childbirth is a big experience and it takes bodies a while to readjust. Plus you may quite possibly have lost some feelings in nerves. And if you've actually had enough of your boyfriend and are looking elsewhere, you're taking on a lot of issues at once, so it's no wonder if you're struggling a bit.
Anyway, you're not alone:
http://news.bbc.co.uk/1/hi/health/2406065.stm