It could very well be a latch or positioning problem, but like Sara said, that can be hard to give advice on over the internet. When DD was a newborn, I don't remember being able to see her tongue between her bottom lip and the breast. I don't really know if there's anything you can do to make her stick it out more, if that's the problem. The nipple sheild may be the cause of her poor weight gain, because nipple sheilds can prevent a baby from enough milk. Also, using a nipple sheild is similiar to a bottle, and how she sucks on it will be different. There's a good tip on this page:
http://www.kellymom.com/bf/concerns/...an-shield.html
It says before each feeding, to put your pinky in her mouth, nail side down and let her suck on it a few minutes. Another tip on there is one all moms can use with thier newborns. If you're holding your breast, put your thumb and finger on the top and bottom of your areola and squish it to be as flat as you can. This helps your baby take as much of the nipple into her mouth as she can.
We used a sheild for a few days and I remember the pain being worse when I took it off. Many moms have pain in the beginning while their nipples "toughen up" as they call it. Using the sheild, you wouldn't have noticed it as much.