Couple questions on your setup:

Did you drill holes in your plastic cage for ventilation?

Do you have a thermometer?

Are you able to measure the humidity? If not you can pick up a hydrometer at a farm/garden store.

You need a thermostat for heat pads. I use UTH's too for snakes. Without a thermostat, I've seen temps hit 100 to 115 degress on my digital thermometer and using my temp gun to double check my temps for accuracy purposes.

What were your temps on the basking (warm side)?