This is completely normal. I believe there is a scientific reason for it, something to do with hormones or chemicals in the brain. It's to help the mom relax as well as the lo. We discussed it in my bfing classes.
It doesn't happen as often to me anymore, but I remember in the early days how peaceful I felt afterwards and how easy it was for me to drift asleep while he was nursing and right afterwards.
I started nursing on our bed with my back against the wall and pillows on either side. That way if we drifted off we were nice and comfy. And like you, it happened often. I loved it. I thin k this is why a lot of moms really enjoy the late night feeds.
Again, this is completely normal and I think almost all moms feel it.... especially during those late night feeds.