Not sure about the STD bit but they have a layer of fat you can feel between the skin and the muscles, I was amazed how chunky and how much movement there is in the skin, specially compared to the corn snake, their scales dont feel quite as rigid and smoother too.
Tig's skin is like that, looser just behind his head and on his belly. The rest of his body is as solid as a brick, but you can still move the skin around a bit. I think as long as it doesn't look loose and rinkly around her sides then it should be fine. But I understand that they can get loose skin before they shed too, sometimes, so could that be it?