You are pregnant

Twenty percent of women spot blood on their clothes during the first twelve weeks of pregnancy. So if you’re experiencing irregular bleeding, it could be a sign that you’re carrying a baby. Blood spots in early pregnancy are usually lighter than your period.
Drugs are problematic
Blood thinners like Warfarin, and in general any blood-thinning medication, can cause heavy periods and bleeding because these medications affect blood flow.
Of course, birth control pills like NuvaRing, which contain estrogen and progesterone, are also culpable. So if you’re starting a new medication, it’s a good idea to read the instructions carefully.

You have a sexually transmitted disease (STD)

STD’s can be hard to spot. You may have one but not know it, because most of these types of diseases are asymptomatic. If you notice blood on your clothes, especially after intercourse, you are most likely infected with gonorrhea or chlamydia. These STDs, as well as trichomonas and human papillomavirus (HPV), can cause inflammation of the cervix, resulting in bleeding.
You are using an IUD
IUD’s can cause some irregular bleeding. About 25 percent of women who use ParaGard (a copper IUD) experience long, heavy periods.
The Minera IUD, which contains Levonorgestrel, a progesterone hormone, can also cause excessive, random bleeding (at least for the first three to six months). The IUD may cause light, short periods, but it can also cause bleeding if it is not properly placed.
You are having a problem with your thyroid
A thyroid problem can be detected from vaginal bleeding. If you are experiencing spotting, it is likely that you have an underactive thyroid. The thyroid can affect a number of reactions and hormones, which can contribute to vaginal bleeding.

You have rough sexual intercourse

A few drops of blood may be seen due to rough intercourse, but this is nothing to worry about.
You have polycystic ovary syndrome (PCOS)
PCOS is caused by the male hormone testosterone.
and can cause problems with bleeding outside of your period.
Testosterone can prevent the normal function of estrogen,
causing symptoms such as heavy and long periods, late periods, and spotting.
Uterine cancer is involved
So far, you have learned about the causes of abnormal bleeding in women.
If you do not find any of the symptoms mentioned in yourself, you should suspect cancer,
meaning that, abnormal bleeding can, in the worst case,
be a sign of pre-cervical or cervical cancer.
Especially if you are currently susceptible to or infected with the human papillomavirus.