Ok you need to go to walmart or target or.. whatever and get a digital indoor/outdoor thermometer/hygrometer (humidity reader) Make sure your warm side temperatures are 92-94 degrees, and cool side is 82-84. Snakes need a temperature gradient to be healthy.

Hot rocks burn reptiles.. they are killers.. NEVER use one. An under tank heater is your best bet, however you need a thermostat for it to be safe. Without a control, pet store under tank heaters will get hot enough to burn your reptile too.. though they are not quite as bad as a hot rock. Expect to pay at least $70-80 for a good thermostat. Ranco or Johnson Controls are good ones. Make sure you get one that is already wired. If you have more money, a proportional thermostat (Helix or Herpstat) are the best. They are around $130 though.

Your snake needs hiding spots too. These are mentioned in the care sheet. A BP without hides is a stressed animal.