Many different things can cause you to gain such as sore muscles from working out alot, hormones, medicine, stress, sodium. etc. As far as sagging skin, no one can predict what it will do. It depends on genetics, age, how much you have to lose, how many times you have gained and lost. Losing slowly, getting in enough cardio and doing strength training exercises can all help. Moistureizing also helps. You are doing great!