It is possible that you are suffering from a mild hormonal imbalance. This can happen if you are under stress, have recently started dieting or exercising or have suffered from illness or any emotional upsets. It can also happen if you have started on a 21 day birth control pack. Most hormonal upsets correct themselves within 3 months. Since yours have continued for 4 months, you will need to take hormones like progesterone for 10 days every month for 3 months (from 16th to the 26th day of the menstrual cycle) to correct the imbalance. You will need to consult a doctor for a prescription.
Monthly blood does not add up. When you have your period, all the endometrium (inner uterine lining) is shed with blood. There is no chance of any old tissue remaining. If it did, you would have irregular bleeding throughout the month instead of every 3 weeks.
I hope this helps.
Your description of your menstrual period appears to be quite normal - many women start off with brownish spotting which increases gradually to brownish blood and then red blood. You are following the typical pattern of the period : light bleeding > heavy bleeding > light bleeding > stop.
While there are no specific vitamins that can correct a hormonal imbalance, it is said that deficiency of iron or Vitamin B12 or Vitamin B5 (also known as Folic acid ) may cause changes in the pattern of menstrual bleeding.
You can try taking supplements of these vitamins and minerals - they may work for you since your menstrual cycle is not irregular, only shorter than the normal 28 days. Vitamin C helps in the absortion of iron and may also be helpful.