Hello and welcome to the board!
I'm not a doctor, I'm just a health nut, but I don't think there is much of a chance of you losing weight so fast that your skin becomes saggy.
When losing weight healthily you skin with adjust with you...make sure you incorporate some type of weight training into your weight loss. When you lift ensure you do light weight for TONS of reps and do isolation exercises.
Here is a handy link:
www.bodybuilding4u.com/weight-lifting.htm
This isn't the only site out there, but it'll get you started...remember: Google is your friend.
-RB
