What might be the cause of continuous menstruation period, e.g. over 15 days stopping for a day & come back the next day. i am 34-yo married, 3 kids.

Many options : There are multiple options. Hormonal treatments such as birth control pills, Progesterone treatments and the Mirena (levonorgestrel) iud are effective reversible treatments that will help many people. There is also a drug called Lysteda that contains no hormones which will help others. If you are done having children discuss the option of an endometrial ablation. In some cases a hysterectomy will be needed.