I think I mentioned it before about the temperature. Sounds like your snake is up to temp and is nice and warm. Any snake in that condition is going to react quickly, and the metabolism will be high so heavy or fast breathing would be normal.

I have and had snakes of many species including many different garters. A snake that is warm and breathing fast, alert, and is nervous is best handled when cool. Opinions will differ but I say cool that snake down for at least 8 hours at night. From your description, the snake just might be too warm. Garters IMHO are best kept rather cool, and will seek out the warm spots or basking areas. Many garters I have kept will hide and get a nasty attitude when it's very warm or humidity is too low. Yet when the air temperature is cool (60-70 degrees) they will come out and seek the basking spot, if it's a basking bulb, heat emitter, local bottom heat, etc. Seriously, I'm not there to monitor your temperatures but it can't hurt the snake to cool it down. I found that one good way to get a garter on a day activity schedule is to cool them down at night. When the heat comes on in the morning, they come out and bask.

May I ask where you got your garter. Forgive me, I have CRS. (Can't remember shi...)

I'm still wondering. Is your red-sided a CA red-sided? Like one of these?
http://www.californiaherps.com/snake...nfernalis.html

I ask because the "original" red sided garter is from the north and eastern part of the continent, roughly.

If you have a CA red side, then WC individuals can be a bit high strung. I have caught large red sides in CA and they put up one heck of a fight. Very spirited.

If you're in Palm Springs you're likely to have indoor climate control that might keep the air a bit too dry. A good heavy mist twice a day with a spray bottle would be good idea and can't hurt, as long as your substrate is dry. Even CA red sides live in low lying wet areas where the humidity is high.