Prior to your problem, were you on any birth controls method?
A brown discharge is usually a blood discharge. If the bleeding is quite slight then it takes longer to reach the outside world and turns from red or black to brown in the process. Such bleeding can be caused by all kinds of things - most commonly infections and erosions (these are raw patches on the cervix).
There are many other possibilities and the only way to find out what is causing it is to go and see your GP and get examined or referred to a gynaecologist for examination.
I suspect this is not going to go away by itself so go and get it attended to. Most of the possible causes are extremely easily treated.
If you think that you may be pregnant, go to your local pharmacy store and buy a pregancy test.
I think it would be best to see your doctor. It could be something simple like your body getting rid of blood clots but it is not worth the risk.
Early Pregnancy Symptoms
The Early Signs of Pregnancy
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Here's another site
Pregnancy Symptoms — Early Signs of Pregnancy
PS: As a woman myself who have had three pregnancies, I have never experienced "black discharge" prior to or during my pregnancies.
However, I have had this type of irregular after my periods (no longer having periods). But in my case some other issues were going on with my body then.