When you exercise, your body produces something called endorphins (it's sort of like morphine), and it makes your body feel less pain and make you feel more happy. In this case, the endorphins combat your stress and make you feel better mentally.
Answer:
Talk to her.
Explanation:
Make sure your approachable and be there for her. Let her know that she can talk to you. Confront her about it but don't press her about it. E.g. pull her aside to where it's just you and her "Sondra I noticed the pill bottle... you know you can always talk to me" or tell a trusted adult to help.
