It's the hormones in coils like Mirena, unfortunately, Jo. They can play havoc with your weight and give you loads of spots and even constant bleeding.
Mind you, I don't know about the not gaining or losing too much weight thing. That could be a problem possibly, too.
My mum and sister had problems with them because they were so painful.
In any case, they sound like horrid things to me, and definitely worth avoiding.