1) there is no harm in fingering your own vagina. Most of your pleasure should come from rubbing your clitoris, or at least rubbing your clitoris while fingering yourself. You can tear your hymen at any time you put something inside your vagina, on purpose or by accident. You can ask a doctor to confirm that you have a hymen, or just use a mirror and look for yourself if you know what to look for. Pre puberty, your hymen would have been much more fragile than during or after puberty when it thickens,pulls back to the rim of your vaginal opening and becomes very elastic.
2) the arousal fluids are plasma squeezed from the cells in your vaginal lining. When they engourge with blood when you get aroused, this process happens, often within a second or two, sometimes even when you are not even aware you got aroused. You cannot deplete it. At some stages in your life and even your menstrual cycle, this will happen easier or more difficult and the amount will be more or less. It can even dry up for a while. Prescription medicine have a big influence on the amount as well. Also remember that you have cervical mucus coming out of your vagina. Sometimes in your cycle there are none, or it may be sticky, creamy, wet and milky or clear and when you are ovulating, a lot of very slippery and stretchy clear mucus (also called raw egg white mucus because of the way it looks and feels).
3) your uterus will move around during sexual stimulation so that can cause some feeling. Touching your cervix at the top of your vagina can be uncomfortable or even painful. Your sexual appetite is closely controlled by your menstrual cycle. So it can also be that you are masturbating at the same time in your cycle and that the slight feeling is natural for that time (for example, PMS or ovulation)
Your feelings are natural and it is happening because of your sex hormones from puberty. Masturbation is the safest way to deal with it. It is healthy and makes you feel good. It teaches you about your body and sexual response and it feels so great. Don't overdo it. If your life is concentrated around masturbation to the point where you do not do things with your friends or family, you should slow down a bit. It is not unusual for teens to masturbate once or twice a day.
Also remember to learn about pregnancy prevention and sexually transmitted disease, and how to prevent it before you have sex with a boy. Once you have done it, it is too late to worry. It is healthy to delay sex as long as possible,butstill make sure you understand the risks. Luck is not a very good way to protect you and your future.