I don't know how long it took you to lose that 20 lbs., but I do know that the faster you lose, the more likely it is that your skin will sag. (I know a woman who dropped a ton of weight in a short period of time and her legs sag like melted wax, even years after her weight loss!) If you do some weight training, or if you're incredibly patient, your skin will "tighten up" after a while, but don't expect to have a stomach you can bounce quarters off of unless you lose a heck of a lot of body fat and build up your abs. (Crunches, reverse crunches, obliques or alternatives if you have a bad back).
At first, my thighs sagged terribly. Each leg looked like a copper beech tree. Two years later, while they're still not as firm as I'd like (still kinda saggy, but that's where I carry my weight, plus I'm in my 40s), they are a heck of a lot better.
Be patient. Congrats on your weight loss and get crunching! BTW --Don't let anyone tell you that the more crunches the better. Doing 100 to 400 crunches a day like the celebs do will not create better abs than doing 25 crunches with more resistance, like lifting a medicine ball when you crunch.)