Sensitivity is a normal side effect of teeth whitening. Whitening products like carbamide peroxide or hydrogen peroxide work by penetrating the enamel surface to get to the discolored molecules.
Your teeth are likely to be sensitive while using any whitening products and for a time after you have stopped whitening. To minimize sensitivity while whitening, I recommend brushing at least an hour prior to whitening and waiting at least 2 hours after whitening. Avoid acidic foods and drinks. Use a toothpaste for sensitive teeth or get a prescription strength topical Fluoride, i.e.- Clinpro. If you are experiencing painful sensitivity, I would recommend taking an anti-inflammatory, i.e. Advil. If the sensitivity persists or lingers beyond a couple weeks after stopping whitening, see your dentist. If you have areas of active decay, the whitening products may cause increased sensitivity. Also, if you notice that your gum tissue is red or irritated from the whitening, you may apply a thin layer of petroleum jelly to your tissue prior to whitening.
You can generally expect the sensitivity to greatly decrease over the next 10 days. I usually have my patients take two 200 mg ibuprofen tablets to decrease sensitivity caused by over-the-counter whitening agents. I also recommend using tooth paste that is made for sensitive teeth. For my patients, a combination of these two things reduces or eliminates the pain drastically over the next few days. Good luck and I hope this helps. Follow me if you have more questions.