I have regularly throughout my life found that yawning can cause cramping of neck, mouth and tongue. It can be excruciatingly painful, bring involuntary tears to my eyes and take quite a while to ease off. Its nothing to do with CRPS - just good old-fashioned cramp and nothing to worry about. I have severe movement disorders which all have fancy medical names with my 'CRPS package' so I know only too well how much CRPS can mess up the way your body's muscles work but the thing you are describing isn't CRPS related. Don't worry about it, just try to yawn carefully and with less gusto!