Y'know I've gotten that rap a lot from certain guys I've been with simply because I'm not the most vocal of people, and I tend to have smaller orgasms over a longer period of time (think small but constant) so while it may not be evident that I'm enjoying myself, I very much am.
The trick here is communication--communicating with my partners and letting them know how much they pleased me let them know that they WERE doing the right things. I am still not very vocal, but they certainly know when I've had my fun now.
You need to sit down with your girl and talk about this--ask her outright if you're satisfying her and if not, what you can do to change it. Tell her you need feedback. Everyone has the right to ask for feedback or confirmation in their sex life. Communication is the key to good sex--when it comes down to it, size is relative, because as long as you can communicate, you know the motion of the ocean.