this actually happened to one of my normals when i fed it small adult mouse. it is kinda big for him, that stayed there for like almost 2 days and just disappeared. i thought it might just be a stretched skin since that was a pretty big feed. i never fed him small adult mouse again after that, i'll wait when he grows larger.
but you fed hopper.. that shouldnt really be that big, unless you got one very big hopper which already happened to me which is almost as big as a small adult mouse.