Gums and teeth throbbing after at home whitening. Is this normal? Its quite painful

After having zoom whitening on my teeth my dentis send me home with a custom made mouth guard and a gel which is 18% and told me to use it everyday for 2 hrs a day for 2 weeks on my bottom teeth. I used it for 4 days and my mouth and gums started throbbing/aching i haven't done it now for 3 days today but its still throbbing and im having to take ibuprofen every 4 hrs