rheostats are pretty much useless. ur hot spot is prolly too hot for ur snake so she stays on the cool side. u can get good thermostats for about 25 bucks and i highly recommend getting one and leaving the rheostat somewhere to collect dust. i had a zoo med rheostat a long time ago and on the lowest setting my UTH was around 110-120 at all times so i chucked it
u may want to check her belly for burns just in case it was too hot for her