During the whitening process your teeth are likely to be more sensitive, some people grin and bear it as it is only short term. The other option is that you can pre treat your teeth before whitening with a fluoride/ desensitising agent this reduces sensitivity during the process. However, first you must find out why your teeth are sensitive and treat that cause first. It could be due to periodontal / gum disease or due to toothwear.

I hope this helps.