Am I being petty? My friend is reaping the benefits of my hardwork?

I don't blame you a bit for feeling as though you are being used. Because, in fact you are being used by your friend. However, she may not be using you for the reasons you believe. It sort of appears she is doing the things she is doing because she wants to be like you and maybe thinks of you as a good example or role model of what she should be like, plus she may also enjoy being with you. If you are really put off by her behavior you could always very covertly locate and engage in these volunteer activities without her knowledge. At some point in time the people in these various organizations will become aware of your true desire to be helping and making a difference and of her just tagging along. Eventually, it will catch up with her. Please keep up your good work regardless of what she does, because in the long run the charities are the ones that come out ahead and isn't that the whole point? Good luck.