I have found that its always good for parents to do some research online and ask other parents about side effects before putting the child on these meds prescribed in great abandon to children for behavioral issues.
I used to babysit a kid that did that, and they cut out dairy from his diet and it helped a lot.
And yes, ADHD meds can sure increase obsessive behaviors in some.
Seems as it is an obsessive thing to do, obsessive behaviors can vere well be co-morbid with ADHD. My son went through the stage around age of 6 where he would have "accidents" with the same babysitter, at her house only. It is certain time of the day, certain place?
Meanwhile, supervise her closely enough to break the "pattern". I really doubt she will look you straight in the eye and pull down her pants.
I think that if you do some research regarding the med she is on you may find the behavior is a side effect of taking these meds that are very powerful..