As a woman, it seems that guys tend to want to switch things up a bit just as they're starting to feel good.. ;-)
If it's tickling her so much that she can't enjoy it, back off, slow down just a bit, and be more gentle. It sounds like you may be moving a little too fast and a little too hard. She needs to trust you and relax to allow you to give her a orgasm.
Follow her lead. If she's not telling you to stop, she's moaning a little, or moving with your hand, then keep doing what you're doing. If she starts giggling, back off just a little.
Try that approach and see how it goes...
Here's a couple of our favorite books on this topic:
How To Give Her Mind Numbing, Leg Shaking Orgasms - The Female Orgasm Black Book Review
How To Go Down On A Woman And Have Her Begging For More