Well your skin has stretched out somewhat and when you lose weight, with it having nothing to fill it, it can sag. It depends on a lot of things like your highest weight, your age, your genetics, etc. Also time can help it as your skin can firm up somewhat over time and will be more noticeable with a drastic weight loss, at least at first. There is nothing you can put on to prevent it though.
This link may be helpful.
Weight Loss And Skin FAQs