Hi Holly, I wouls assume you are just having very heavy periods which can make you very tired and run down,. It may be a one off or you may need to go and see your doctor, doctors are very used ti this problem. Below is a little info, but do go and see your G.P..
Symptoms
An average amount of blood loss during a period (menses, menstruation) is 30 to 40ml (six to eight teaspoons). Measurement of the exact amount of blood lost is very difficult. (Researchers have to weigh new and used sanitary towels, but this is not the sort of thing most of us would want or need to undertake!)
When you are losing excessive amounts of blood, you may keep passing large clots (like liver) and you may need to change sanitary towels or tampons very frequently.
Women vary in how long it is from the start of one period to the next. In some it is less than a month and others it is longer. Some have an irregular pattern. The actual length of the period varies, too, and may be from three to seven days. In menorrhagia, some women have very prolonged blood loss, with only days before the next episode.
Heavy periods may be accompanied by cramp-like period pain, but some women find even their heavy periods painless. (The medical term for painful periods is dysmenorrhoea.)
(Heavy periods (menorrhagia)
Heavy periods (menorrhagia) affect many women. However it is difficult to be sure what people mean by heavy periods. Heaviness of periods is very subjective. As with many other bodily functions, that which is considered perfectly normal by some, might be thought extremely abnormal by others
Persistent heavy periods can lead to thinning of the blood (anaemia), which can cause tiredness, shortness of breath, faintness, and even angina. Symptoms of this sort would usually prompt people to see the doctor anyway.
Hope this helps.