I've taken a lot of SOMA over the year without any side effects but a wonderfully pleasant drowsiness and some welcome relief from back spasm.
It sounds like you are having a SLIGHT reaction to it (taking anything else?...niacin?...strawberries
A SERIOUS anaphylactic reaction (hives, lip-swelling- throat closing angioedema, shock or coma) usually comes on fast.
I'd mention it to your doctor...maybe a couple aspirin and some benadryl or prednisone (if you have any lying around your medicine chest) til you get him on the phone.
I wouldn't worry though. A SERIOUS reaction would have hit you last night.
Personally, I doubt the SOMA (carisoprodyl) has anything to do with the rash. (I LIKE the drug!)