In my experience, and observing those of my friends, I have found that SOMA (Carprisidol) has a high-rate of allergic reactions in people, i would say somewhere around 10%.
If you are able to come back and read this post, obviously your reaction wasn't anaphalactic, but it still sounds like an allergy none the less. If you are prescribed these, I would stop and call your DR.
If you are getting them on the street, I would especially stop.
If you insist on continuing to use them, keep plenty of benadryl handy (liquid form would be best since if you do have a bad reaction it will be hard to get pills down) and go to the pharmacy and get a Primatene brand inhaler - it is just an EPI inhaler which can be used similar to an epi-pen in emergencies, it is available OTC without a script. There is always one in my backcountry first-aid kit along with an epi-pen (although these are hard to get if you don't have a wilderness EMT license)
Keep in mind that your reaction will get worse (if there is one) every successive time you take the drug that is causing it.