Don't worry, it's just milk chocolate. If it were a large amount of dark chocolate or cocoa beans it would be a lot worse. It's the antioxidants that are what really harm the dog, and there isn't much antioxidants in a Hershey's chocolate bar. Most of those antioxidants in the chocolate were boiled away when the cocoa bean was heated to make that chocolate bar, then doused with milk and other ingredients. So your puppy will probably just get a tummy ache, and most likely either throw up or have a really nasty poop. Just be very gentle with her; try not to play with her too vigorously for the rest of the day. Treat it as if she is a little kid that got in to too much candy; expect a couple of messy accidents.
I know everyone says "no chocolate; chocolate kills dogs" but this is not true. I have seen and heard of many dogs getting in to chocolate and being fine. Some don't even get tummy aches.
My moms dog ate an entire 8x12 pan of brownies and he survived. He was super hyper for a while, then had a nice big nap and poop and he was perfectly fine!
Don't worry unless she starts yelping as if in agonizing pain. Then seek a vet; but that is not likely to happen. Make sure she drinks plenty of water.