I use Crest whiting strips and now my mouth is burning.
You can be sensitive to the strips more so then others. Taking them off sooner then the full 30 minutes was probably best thing for you to do. Whiting strips can hurt your teeth because of the high amount of hydrogen Peroxide, try using strips with low dosage. If your gums and mouth still hurt see your doctor. You might be having an allergic reaction to the strips, in which this case they can treat you for that.