Her Kegel muscles (muscles along the pelvic wall) need exercising. Kegel muscles control (among other things) the stream of urine when going to the bathroom. So, if she does her Kegel exercises (pretending to stop the urine, letting it go, stopping it, etc), this can make the vagina tighter.
She can do it while sitting in class, laying in bed, during dinner...seriously, ANYWHERE. It can be extremely beneficial during sex to do these practices. Doctors tell women they should do it after childbirth to regain control of the urinary tract and to just regain the strength of the vagina.
Shouldn't be anything problematic.