By breathing heavy, if you mean you can noticeably see her sides going up and down, then that can be caused by heat. I've seen that with some of my balls and corns when they are laying on the warm side.

For a cheaper thermostat, you can try the hydrofarm off Amazon. I've used them and they work great in a pinch.

http://www.amazon.com/Hydrofarm-MTPR...I2HKWS61XS2CQV

Just order with next day shipping and you should be ok. To make sure that your tub isn't getting too hot, you can always raise the tub off the heat mat a bit so it heats the air. It's not going to be perfect but less chance of burning the snake since you don't have a thermostat.

Sounds like she's getting better though which is a step in the right direction.