Is it douching that causes my menstruation that does not stop?

No. Douching, when performed normally, should not affect the length/duration of your period .
Not likely. Douching can wash out old vaginal blood from menstruation, however a continuous period as a result is not likely. By the way, 'lose the douching', as the vagina doesn't need it and you can cause more serious infections or problems.
No. Douching only affects the vagina. The vagina is really a pouch of elastic tissue. At the top of the vagina is the cervix and the uterus. Menstruation comes from the uterine lining shedding and only passes through the vagina. If you are having prolonged menstruation, see a doctor as this can sometimes be hormonal imbalance, thyroid problem, fibroid, polyp or even pre-cancer.