These pains are the ones we normally classify as "period pain." Clue explains that they are "common pain symptoms in the days before and during menstruation." The phenomenon is caused by cramping of the uterus as it begins to "pulse" to get rid of its uterine lining; while the uterine tissue itself doesn't cause pain, the process of getting rid of it does. "Scientists suspect that cramps may be caused by an excess of prostaglandins, which are hormone-like substances that help the uterus contract to shed the uterine lining," Clue's scientists tell Bustle. However, the fact that we're not entirely sure about what causes a very common and benign aspect of the menstrual cycle shows you how tricky menstrual science can sometimes be.

Pain related to your period doesn't have to be in the lower half of your body; it can also hit your head. If you get serious headaches or migraines around your period, they may be related to your hormones. "Many people with migraines report an association between menstruation and migraine incidence," Clue's scientists tell Bustle. "People with migraines also report changes in migraine experience throughout their reproductive lifespan relating to first period, oral contraceptive use, pregnancy and/or menopause. Changing estrogen levels are believed to mediate these changes and taking extra estrogen (as hormonal birth control or hormonal replacement therapy) may make migraines worse." This isn't viewed as problematic, necessarily, but the Clue experts note that it's a good idea to track the incidence of headaches and migraines in relation to your cycle so your doctor can help you figure out whether hormones are causing an issue.

The Germans have a particular word for this specific menstrual cycle pain: mittelschmerz. And it's associated with the ovulation window of your cycle, not your actual period. "Around the time of ovulation," Clue's scientists tell Bustle, "some people recognize “mittelschmerz” — or ovulation pain — a sharp or cramp-like pain felt on one side of the lower abdomen. It can appear suddenly and subside within hours, or it could last for a couple of days." While the cause of mittelschmerz isn't precisely known, it's suggested that fluid and blood released with the egg as it travels out of the ovary might be irritating internal tissue in the rest of the abdomen. If you experience mittelschmerz, it can actually be a good clue to where you are in your menstrual cycle, but it's not a good enough guide to replace actual birth control.

If your menstrual cramping is debilitatingly painful, it may signal that you're suffering from a more severe issue: endometriosis. "Endometriosis," the Clue scientists explain, "is a common (and under-diagnosed) cause of painful menstruation." Endometriosis sufferers experience the growth of endometrial tissue — aka, the uterine lining — on other areas of the body. That tissue behaves just like the tissue inside the uterus, shedding and regrowing every month — a process that can cause internal pain, scarring, or even infertility. The "shedding" period, up to and during menstrual periods, is often excruciating for women with endometriosis, and it also often gets worse over time. If your pain is significant and debilitating, you need to see a doctor; while there is no cure at present for endometriosis, there are a number of therapies that are recommended to manage it.

Unexpected pain before or during a period may also point to a problem with an ovarian cyst. Two types of ovarian cysts exist: follicular cysts, which form on the ovary if it fails to break open and release an egg, and corpus luteum cysts, which form after an egg has been released and the follicle fails to go back to normal, instead swelling with a bit of fluid and blood. Many women make at least one cyst a month, according to the U.S. Department of Women's Health. Ovarian cysts often don't cause many problems on their own, but they may cause period pain around the menstrual period — and if they've ruptured, they may cause pelvic pain at any point in the cycle. The vast majority of cysts are viewed as normal, healthy consequences of the ovulation process, but a spike in pain needs to be assessed by a medical professional.

Worsening menstrual cramps seem to be linked in some women to higher levels of smoking. A study in 2014 which tracked 9,067 women for around 13 years found that the more they smoked, the worse their menstrual cramps were, and that starting smoking early in your teens seemed to be a significant factor. Understanding why is still elusive, but some scientists think it's because of lower available oxygen levels in the body; cramps tend to cut off oxygen supply to the uterus, and smoking's constriction of blood vessels might make this process worse. Another reason to quit.

Some period pain isn't really related to periods at all; one potential culprit, which is accompanied by other symptoms but primarily causes really bad pelvic pain, is pelvic inflammatory disease, an infection of the reproductive system often caused by a sexually transmitted disease. Pelvic inflammatory disease likely isn't behind your horrendous period pain if you don't also have a fever, discharge, or painful urination, but if all of those problems show up, you need to go to a doctor as soon as you can.

Uterine fibroids are relatively common, but not all women who have them experience any symptoms. Those who do, though, count unusually heavy and painful periods among the problems. Fibroids are non-cancerous, but they can be very annoying: they develop around the uterus and are composed of muscle and tissue. Fibroids can cause more pain during menstruation because of various factors: they can increase pressure on the uterine lining and produce more cramping, and their tendency to have blood vessels means they can also create increased blood flow. They can press on nerves that create pain in the pelvic region and lead to inflammation of the uterus, all of which is a recipe for agony.