Skin getting loose will happen ... Nothing to worry about just fill it out. The pulling exercises like hanging and such will cause this but it will equal out just make sure you have enough variety to your workouts.
Also if your skin is getting sucked up into the
bathmate, try pumping with a cock ring. One thing I do to keep my skin tight while pumping is I pump with a condom on. Try that, you will likely get better pure expansion anyway because this will keep the skin tighter to the tunica which will give less room for fluid buildup.