What Ranai said is very wise. First and foremost be up front with eachother. And if she knows how much you enjoyed it, she might be will to do it again.

Maybe you could encorporate this into a 69, and please her while she's doing it. Plus, speaking form experience, it is totally mind blowing to get a blow-job while the girl is massaging your prostate.

A bit off topic, but you might want to get one of these; it's called the Aneros, and I was recently given one by my Domme (as some people can attest to, I have been talking about it quite a bit.) This thing is amazing, it's not much bigger than a finger, but the design is brilliant. Basically, when your muscles contrat, it will massage your prostate, so it's totally hands free. This might be a good thing to introduce, that way, you could get the stimulation you want, but your girlfriend wouldn't have to do something she might not like. And since it's hands free, she could be doing other things to you, you could even use it during intercourse.