Faye, Remember that your parents are your primary resource for information about sex and what is happening to your body. If you are not comfortable talking to them, find another adult woman you trust, a teacher or a nurse or an aunt or one of your friend's mothers.
The time you start to menstruate is called 'menarche'. Halve of girls in the USA/UK reach this before age 12 years 5 months. The other halve after that. You do however reach puberty about a year before that when your breasts start to enlarge and you start to grow pubic hair. From the time your puberty start, you can have wild fluctuations in your hormone levels which can cause all sorts of urges.
Many younger girls discover masturbation and the wonderful feelings it can bring at a very early age, just like you did. Unborn babies even masturbate and orgasm in their mother's wombs. This can sometimes be seen on ultra sound scans. Masturbation is thus totally normal and natural. You are born with the instinct to orgasm.
Since masturbation does not harm or hurt you, it is difficult to say it is harmful. As long as you do not become preoccupied with masturbation and do it in privacy, it is not unhealthy. Even though we do not talk about is, most women masturbate from time to time, your mother, aunts, cousins, friends, teachers, friend's mothers, shop assistants, police women, soldiers, nurses, doctors. It does not matter how old or where in the world. It is happening all around you.
What your parents did is a normal reaction to finding their children masturbating. What they should have done was to educate you about what was happening with your body and what is acceptable behavior for a young woman and what is not. That way you can make your own decisions based on your knowledge, not on old wives tales and scary stories, or the fear of punishment.
As a society we should be teaching young adults that sex is good and pleasurable. With sex we do however also get responsibilities; like controlling our urges, not becoming slutty women, putting our health and well being first when it comes to sex, reserve sex for long term relationships, to take responsibility and prevent becoming pregnant outside our long term relationship, to treat other people ourselves and our bodies with respect. Unfortunately we more often teach teens that sex is shameful, bad and wrong.
This is one of the reasons why you feel 'dirty'. The other reason is that when you have an orgasm, loads of feel good hormones are injected into your brain and blood stream. You get a 'high' and feel healthy and great. As soon as these hormones leave your body, you go into a state of slight depression that makes you feel guilty and ashamed. This is normal and all men and women feel this way until they make peace with their sexual side. And even then you can still sometimes feel this way.
It is good that you also use your hands. I would encourage you to rather use your hands and fingers to masturbate; this will allow your body to become used to the more focused stimulation you will typically experience with a partner later. Turn around on your back and reach orgasm that way.
Orgasms are a great way to relax, get to sleep, work off stress, learn about your body and sexual self. Like with everything else in life, too much is bad for you.
What is the secretions you find inside your knickers? Your whole fertility cycle is controlled by your hormones. These hormones also cause various secretions that come out of your vagina. When you start to menstruate, these hormone cycles will become much more pronounced. Your cervix at the top of your vagina will secrete all sorts of different fluids during your cycle. Menstrual blood, a sticky whitish fluid, a creamy fluid and a very wet clear and stretchy fluid. Some days it will be completely dry. During puberty, your hormones are not fully settled down. These fluids can appear in copious amounts as your hormone cycle starts up and begin to regulate.
When you become aroused, blood flows to your vagina and vulva. Your clitoris becomes erect and your vagina swells up. This swelling squeezes a fluid from the cells in the vaginal walls. This is called arousal fluid and will also come out of your vagina onto your knickers.
Talking of secretions, remember to wash your vulva with either a mild unscented soap, or just plain clean water. Wash between all the folds down there. Also wash under the hood that is over your clitoris. Just pull it back and wash there.
I hope this answer the questions you had!