Pain is your body's way of telling you that you're causing damage. Stop before you rip soft tissue and cause problems. Assess the situation, ask her how she's doing, how it feels if you just hold still, if she needs you to come out, or what she'd like you to do. More lube doesn't hurt either - you really can't have too much. If your partner is lubed, aroused, wanting more penetration, and it still hurts her when you try to penetrate her more or with more of your hand, make sure you're not pushing against her urethra.
Discretion is the better part of valor.
And you know, that's really quite okay. Fisting isn't for everyone, and there are plenty of other ways to have amazing sex. It's not a competition, and you aren't any less of a woman if you can't be fisted or just don't really want to be. There are a couple of other times when fisting may lead to pain, either for the fister or the fistee. Every once in a while, particularly when a woman is being fisted on her hands and knees, the insertive partner may possibly accidentally hit, tap, or bump an ovary through the vaginal wall.