A good thing or a Bad thing!?!

The girl I have been seeing for about a month says "she gets really nervous when she has sex with me" and that she "can't understand it, it's like her first time." (she's 26 years old)

She cums several times but only through penetration. Every time I go down on her and attempt to stack her orgasms she stops me when she gets close.
So maybe I haven't made her feel comfortable enough, but every other part of our relationship is comfortable.

Any ideas on what I can do to help her get over this?