You don't have to feel ashamed -- it's perfectly natural. An expert tells us why.

[In a human sexuality class], I asked my professor about this experience. She confirmed that it is possible and probably quite common, although people have feelings of shame about it. Pleasurable breastfeeding was probably one of nature's ways to make a survival behavior attractive.

The mental boundary was so great that I was convinced it was impossible. Then one night while I was sleeping, I had one of those fabulous dreams that, if a guy had had it, would have involved moisture. You've heard about them. I woke up from the dream to find that my tiny vampire had been doing some nocturnal suckling while I slept.

http://www.momlogic.com/2009/09/women_who_orgasm_during_breastfeeding.php?page=3

(http://www.autoadmit.com/thread.php?thread_id=2544822&forum_id=2#25393879)