Getting an iud & gaining weight

So can someone please tell me what is it about getting an IUD that makes you gain weight because I guess I understand birth control yes but I also eat moderately healthy so do you really think it’s just what’s in the mind that makes you think that you’re still eating healthy even though you probably really not and you blame the IUD for why you gain weight tell me I’m wrong because I know that when I keep track of what I eat and what I don’t eat I feel fine I don’t feel as heavy even if I’m on my birth control and I lost weight but for some reason it’s just different. Because I gain weight when I don’t eat good and I feel more lighter when I eat better should I blame my birth control for how bad I eat