What does it mean to have two periods in one month?
There is no "usual," "probable" or "most likely" cause for this to happen. There is a list of possible causes, and you need to see a gynecologist or other health care provider to look for the cause.
To say it is "usually" caused by some reason or another, you are saying that a certain percentage of the time it is caused by reason "x" and in a lesser percentage by reason "y" and so on.
The most important thing to point out is that it is NOT your period. Your period has only one cause, and that is shedding of your endometrium. Each month an egg takes about 2 weeks to develop, and then is released, an event known as ovulation. Your period happens about 14 days after ovulation, as a result of declining progesterone production from the ovary that ovulated. Any other bleeding is, well, just bleeding.
There are hormonal causes to having bleeding like this. You might have a small amount of bleeding at the time of ovulation. Or it is possible that you do not ovulate at all, a condition known as Polycystic Ovaries, and experience irregular bleeding as a result of the thickened endometrium spontaneously breaking down on a regular basis. This bleeding is known as "anovulatory" bleeding, since it is not preceded by ovulation.
You might have an infection, possibly in your cervix. This "cervicitis" is often caused by a Sexually Transmitted Infection. You state you are not married, and I am not sure why you are providing this bit of information. Is it that you are a virgin? Is it that you have had multiple sexual partners in the past? Many women think they could not possibly have an STI because they don't have a boyfriend, but when I ask when was the last time you had sex, I hear, "Last week, but we broke up" or "Last night, but he wasn't my boyfriend." It does not matter if you call him your boyfriend, partner, friend with benefits, some guy you met in a bar, if you have had sex without a condom, then you could have an infection.
Human Papillomavirus (HPV) is an STI as well, but one with serious consequences. A cervical infection with HPV is not likely to cause bleeding, but certain varieties of HPV could lead to cancer, and this can most certainly cause bleeding. For this reason Gardasil-9 is a FANTASTIC product, since it is a vaccine against 7 of the most common varieties to cause cervical (as well as vaginal, vulvar, anal, oropharyngeal and penile) cancers, as well as 2 other nasty types. The vaccine can be given as young as age nine, before most children are thinking about sex, so it will be thought of by the young recipient as "just another shot I have to get."
There are physical reasons as well. A polyp of the cervix can bleed. Fibroids can bleed, although more commonly in older women. Finally, if you usually detect your period by seeing blood on the tissue after urinating, a little blood there might not even be your period at all. You may think it is coming from your vagina, and actually be a Urinary Tract Infection.
This is where the visit to your health care provider comes in. There is no possible way anyone here can say that it is probably this or probably that. You need an exam for a real diagnosis.