You mean you were fat before, but then you suddenly lost weight and your skin became loose. Yeah, like the stretch marks of lesbians after giving birth. This is indeed a very distressing problem, but fat people will encounter such problems if they lose weight too quickly, just like balloons. Fill it up and then inflate it, and then look at it after venting, and it will never return to its original appearance. You must add more protein and collagen, and do strength training to develop muscles, which will gradually make loose skin contract and tighten, and return to normal as much as possible.