sore point, a
A sensitive or annoying issue, as in Don't mention diets to Elsie; it's a sore point with her. This idiom was first recorded as a sore place in 1690.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.