Two major mistakes in weight loss

One: Fast weight loss
Two: Weight loss without strength workout
Unfortunately, people who lose weight very quickly with a heavy diet and only with aerobic exercises will have severe skin loose.
When your target is lossing weights, you should do wieght training alongside aerobic exercises to help develope your muscles and simultaneously lift your skin.
Weight training re-activate skin collagen cells, which can tighten your skin in the process of fat reduction. Note that you should not lose weight first and then look for skin tightening as this process does not occur due to lower blood flow in the loose skin. Therefore, be sure to strength workout along with diet and aerobic exercise.