My daughter also runs cross-country.  While she doesn't seem to have this problem, I'd say at least half the team does, based on the number of Depends handed out as joke awards at the end of the year, and the number of discussions I've heard about the best color of shorts for your uniform - (black is preferred and light colors are NOT).

If it really upsets her, you could have her start doing Kegel exercises (squeezing the muscles down there - try it while urinating to stop/start the flow to get the right muscle group).  About fifty, done several times a day, may help with this (no guarantees though).

My daughter has had this happen to her two times now.  And I wonder if wearing poise pads would help.  I'm thinking about having her to try using those.

see attached article