Vaginal discharge is a completely normal physiological phenomenon, and many women notice brown discharge right at the start of a menstrual period and as their period comes to an end. This, too, is normal. You can think of brown discharge at the beginning of your period as a sign you're about to menstruate, and brown discharge at the very end of a period as a sign that your body is clearing the last of your menstrual fluids. 
What are the possible causes of brown discharge between periods when you are very sure you are not pregnant? Before we start looking at that, you should realize that brown discharge signifies the presence of old, dried up blood. What we're really looking for, then, what can cause you to experience spotting between two periods.
Birth Control Pills And Other Hormonal Contraceptives Can Cause Bleeding Between Periods
Hormonal fluctuations are a frequent cause of spotting after one period and before the next one — and if you use hormonal contraceptives of any kind, you have your main suspect in your pocket already.
While this is not something you should worry about from a health perspective, bleeding between periods is a common reason for which women opt to switch to another birth control method.
The Cause Of Your Bleeding After Periods May Not Be In The Reproductive System
Hypothyroidism, or an underactive thyroid, can cause spotting between periods, along with diabetes, coagulation disorders, and liver or kidney disease . When you experience abnormal uterine bleeding or spotting, it's natural that you're wondering if there is anything wrong with your reproductive system, but that is not always the case.
Intermenstrual Bleeding Due To Pelvic Inflammatory Disease And Other Infections
Pelvic inflammatory disease, an inflammation of the reproductive organs that can encompass the uterus, fallopian tubes, and ovaries, is usually — but not always — caused by a sexually transmitted disease. Untreated gonorrhea and chlamydia have a high chance of leading to PID, for example.
Could Uterine Polyps Or Fibroids Be To Blame For Your Spotting?
Uterine polyps and fibroids are different kinds of abnormal growths that form inside the uterus, and both can lead to menstrual cycle irregularities and bleeding between periods. Uterine polyps can lead to both cancer and infertility. If you notice pain during sex, bladder discomfort, and abnormally heavy periods as well as spotting after your periods are finished, please consult a doctor as soon as possible. 
Simpler Explanations For Your Bleeding After Periods
Could brown discharge after a period have a simpler explanation, too? You bet!
Small injuries to your vagina or cervix — caused by passionate sex, varicose veins, an ulcer, or genital warts, for instance — can cause spotting as well. If it takes a while for the blood cause by those injuries to leave your vagina, it may well manifest as brown discharge. People who are on blood thinners are more likely to experience this kind of spotting. 
Brown discharge is also more common in post-menopausal women, who frequently experience vaginal dryness as a result of their lack of estrogen  and are thus prone to vaginal micro-injuries.
Finally, though there's little scientific basis for these explanations, it is possible that your brown discharge is the result of a so-called "ovulation bleeding", something some women report they experience shortly after ovulation, or due to an "implantation bleeding". The latter sometimes takes place when a fertilized egg nestles into the uterine lining very early on in pregnancy.
The Bottom Line
Women who consistently experience brown discharge, or heavier spotting, in between periods can pick and choose the cause they perceive as most likely from our list — but that won't give them any real answers. Because some of the causes of intermenstrual bleeding are serious, it is important to talk to your doctor about your brown discharge if it happens, so you can stop speculating and find out what your brown discharge really means.