Like a lot of Moms have said.. you don't want to come slamming down on them and cause a lot of shame for what is just natural curiosity. This wasn't hormonal, dirty, or perverted - just innocent curiosity about things that are kept covered. There are more nerve endings in the most private of areas - even to children they notice this.
This is best time for you to start establish open communication with your child. Maybe time to buy a book about body differences and answer any questions your child has honestly. While you're doing that you can explain to her that her body is her own. You could tell her no one else should be touching her body's private parts unless it's a doctor and Mommy or Daddy are there.
It's not wrong to be curious. If you make her feel awful and scream, yell or punish you are slamming the door on any hope of her coming to you if something else HAPPENS. She will have an established feeling of "shame" about bringing things to you.
Honestly I tell my son, 5 years old, when he touches himself, "I know that feels good - and it's okay to touch, but that's something you do in private." (You know boys, they start grabbing at Thanksgiving dinner -- whenever the mood strikes and my boys figured out where *that* was nearly as soon as they got the muscle control. Diaper came off and hand immediately went down.)
It's perfectly normal. :) Good luck!