His and Her Health

my girlfriend cums within two to three minutes of penetration and then asks me to finish because after she cums she says her vagina feels 'numb'...this is kind of frustrating because while it lasts its amazing and i know i could go on for far longer if she didnt have that discomfort after the first orgasm.
its interesting because this is our second time around as far as our relationship goes...during our first relationship, which lasted for about a year and a half, we used to have long sex sessions during which she'd have multiple orgasms. she revealed to me recently that she had this same numbness after her first orgasms then, too, but she let me continue because she felt bad to rush me out; eventually the sensations would come back and she'd experience orgasm again.
but i guess she doesnt feel the subsequent orgasms are worth waiting for through the resulting numbness...
i guess i'm wondering two things; what is this numbness that she experiences after orgasm, and is there a way that she can learn not to orgasm so quickly so sex can last longer?