First let me say it shows a lot of dedication on your part as a parent to want to do something to help your daughter. Congratulations! You must be a wonderful parent.
Your daughter has discovered that 'it' feels good. Her hormones are pushing her and very hard. It's past the time for the discussion about sex, and it's time to start really talking about the responsibility of coming into her sexuality and taking control of it.
Start with helping her to understand that she isn't bad for having those kind of desires. They serve a purpose. If you need to help her understand that purpose, I'd like to suggest the good old standby "NOVA - Miracle of Life" video. You can watch it on line for free at this link.
Your families religious/moral beliefs will also play an important part of this discussion. Masturbation is a very difficult subject, but if you don't explain it, and it's safe limitations (morally, sanitary and appropriate or inappropriate place, time, etc.) she will just learn to hide it from you. Start the discussion. Keep it open for a while before you start to guide her with your feelings on the subject. Don't tell her that her sexual feelings are wrong, but gently guide her intoCustomerthat they have a place and that she has control over them.
Let her know that there are some serious disease issues surrounding masturbation, beastialtiy and sex. Some kids mistakenly think that if they aren't having sex with another person, it's safe.
There is also a great amount of physical energy built up around sexuality. If she is overstressed at school or in another situation she me be finding relief through masturbation. Help her to find a different release. Exercise, join a gym, sign up for a class learning a new hobby. Keep her out of the house, and keep her busy. Monitor what she's exposed too on TV and the internet. But, don't let her know that your intent is to keep her mind off of it.
Stay completely involved in her life, as the caring parent that you are, and she'll be ok. Don't change how you love her, this is just a hard time in her life, and she's learning new boundries.
I have the very deepest regards XXXXX XXXXX Continue doing what your doing!
Mom to a 13 yr. old girl, and 15 yr. old boy