I just had my teeth whitened, and the sensitivity is really bad. How long is it going to last?

I got my teeth whitened in-office today. It kind of burned while I was getting the treatment, and four hours later, my teeth are really sensitive. They hurt even when I'm not eating or putting any pressure on them. Part of the gums were white earlier, but that has gone away. It feels more like an ache than a burn, so I don't think it's a burn from the bleach, but I'm not sure why it hurts this badly.