A woman's nipples can be wonderfully sensitive. Of course, it all depends on the person and also, ironically what time of the month it is (I know, I know you guys don't want to think about that, but it's true !!)
And based on the fact that she begged for more, you'e stumbled onto a wonderful little secret. If she's anything like me, she'll like having them lightly nibbled on, heh heh.