If you notice the scar on her back this is due to
the extensive surgery she had when 12 years old to try to straighten her back from Scoliosis. The resulting operations would have left her in pain for many months and i would think still to this day.
http://www.rnohcharity.org/the-appeal/princess-eugenie-s-story
Maybe (double meaning ) Potus was referring to her health battles and overcoming them.
Just a thought