when you lose weight, the skin can sag if there is no body toning done, like sit ups for abs, push ups to help arms, leg up, for legs etc.
the best way to lose the saggyness is to do some weight training, it strengthens your arm muscles, and will tone your body and make the skin tighter.
when weight is lost fast, it can leave skin lose afterwards because there is no muscle now.
maybe you could get some 5lbs. and 10lbs. weights and do some weight lifting at home, and some other exercises for the rest of your body. there's a bunch of different kinds on the internet to look up. hopefully that helps,
or i just thought now, my boyfriend was telling me when he was younger, he would play in the mud with his brother and they would end up covered in mud then lay out in the sun, and the next day their skin would be tight and firm.
now, im not saying play in the mud, cover yourself and bake lol, but you could go to a spa and take a mud bath if it helps.

i hope this was a little helpful for you.