Most Helpful Girl
Dramatic weight-loss can cause saggy or excess skin, thus it's suggested to lose weight at a moderate tempo. I assume she lost a lot of weight or lost it very quickly. Unfortunately exercises and non-invasive treatments only work with minor cases.
In the case where the excess skin is quite severe I would suggest plastic surgery. It is a procedure called body contouring and can be quite expensive and invasive. Do some research on the operation and then decide if the skin really bothers you enough to make your girlfriend have a major surgery to change it.
If you decide this is what you want your going to have to introduce the idea very subtly. If she has just lost a lot of weight she will also be keen to look her best, so watch plastic surgery shows or bring up a story of a friend of a friend who had the operation. It will probably hurt her feelings a little, especially coming from you. If this is to hard for you try talking to a friend of her and ask if she would suggest it. It is better hearing it from a friend than a boyfriend.
Hope this helps, but remember don't base a relationship on trying to change your partner. It always ends messy.