Periods is a natural process that starts once a girl attains puberty and her pituitary-hypothalamic axis gets activated. The hormone estrogen is secreted from the ovary and acts on the inner lining of the uterus to prepare it for pregnancy. When the pregnancy does not take place, levels of hormones progesterone and estrogen drop and menses take place.

The causes of irregular periods are- hormonal imbalance, stress, thyroid issues, prolactin level issues, PCOS, and many more. You need to consult a gynecologist for checkups and further treatment.

The reason for prolonged periods could be pregnancy-related like incomplete or missed abortion or hormonal imbalance. Consult your doctor at the earliest if your periods are prolonged for more than 7 days.

White discharge is normal if it is not foul smelling or associated with itching or if the color of discharge is curdy white, grey yellow green, or red. Also, the timing of the discharge with respect to your periods helps in diagnosing the cause. Normal discharge happens because of the hormones estrogen and progesterone. It helps to keep our vaginal moist and also helps in cleaning our vaginal and keeping it healthy. Sometimes reddish white discharge during 12-14^th day of regular periods could be due to ovulation. However, if the discharge is altered in color or associated with any of the above-mentioned symptoms it could be due to vaginal infections like fungal, STI or due to polyp (if reddish) or cervical cancer (usually in older age groups), and many other causes. A proper physical examination and history by a gynecologist would help you to know the cause and get treated for the same at the earliest. 

The hormone estrogen is secreted from the ovaries once puberty is attained. Estrogen causes fusion of the epiphysis of the bones and thus the physical growth stops after periods.

Medicines should not be taken for any cause without consulting your doctor.
If bleeding is heavy and prolonged for more than 5-7 days, medicines can be prescribed by the doctor. Taking medicines to postpone or pre-pone periods sometimes due to some family functions or other is ok under the supervison of the doctor.
Yes, you can take medicines for periods of cramps. However, you need to be investigated for the same as having pain during periods that affect your daily activities is not normal and acceptable. 

Periods will stop probably after the age of 45 years to 50 years. That’s called Menopause. All women undergo this. As the follicles in the ovaries reduce and become almost zero, menses shall stop permanently.

Deranged thyroid levels affect the flow, amount, and duration of periods. It can cause scanty periods or heavy periods or delayed periods or no periods for months.

Because of hormonal imbalance that takes place.