It is not abnormal, but if you feel guilty about it, it is obviously hurting you. Some women masturbate a lot, some do not at all. As long as you do it in a healthy way, it is ok.
Masturbation is natural and healthy for you. At your age it gives you the safest possible way to deal with your sexual urges. If it interferes with your daily routine however, you should cut back on the number of times you do it.
If you look around you, most women you see masturbate. your mom, sisters, aunts, teachers, friends, girls at school, women you see in shops and on the street. Even babies in their mothers wombs have been seen masturbating on ultrasounds. So if you feel guilty, deal with it. Find out why you feel guilty, and then just stop feeling guilty. Masturbation is a private thing, but it is not something to feel guilty about, or be ashamed about.
Take care!