“Uterine lining builds up over the course of your cycle, and the uterus responds with cramping as a method to control the bleeding [during menstruation],” says Christine Masterson, M.D., chief of the women and children’s service line at Summit Medical Group in New Jersey “If you have a lot of menstrual blood or a blood clot, sometimes the uterus will cramp to expel that.”

Most of the time, painful period cramps are super inconvenient, but pretty much NBD...until they're not, like if they affect your daily functioning, says Masterson.

So how can you tell the difference between normal period pain and something more serious? Unfortunately, it’s pretty subjective: Masterson says that everyone has a different tolerance for pain, so there’s no exact number on the pain scale to indicate that your individual cramps have reached a “red flag” level. But if you can’t get out of bed to go to school or work, can’t find relief from any OTC pain meds, or have any of the six types of cramping listed below, you should talk to your doctor ASAP.

It might be: . These benign growths on the wall of the uterus are common, says Masterson, but they increase the surface area of the uterine lining so the amount of cramping and bleeding you have during your period may become super-intense.

What to do: See your doctor, especially if you know other women in your family have had fibroids (Masterson says there can be a genetic component to these). You’ll likely be sent for an ultrasound to make sure there are no abnormal growths, and then prescribed  to minimize the pain during periods. Depending on the size and location of the fibroids, you may also be a candidate for surgery, according to the National Institute of Child Health and Human Development (NICHD).

It might be: pelvic inflammatory disorder (PID). “PID is characterized by constant pain outside of your menstrual cycle that comes with vaginal discharge,” says Masterson. The condition is a serious infection of the uterus, ovaries, and/or fallopian tubes often caused by untreated STDs like  or gonorrhea.

What to do: Get to your doctor, because you need a swab culture to check for bacteria or an infection, says Masterson. PID is totally curable with antibiotics, but if left untreated for too long, scar tissue could form in your reproductive tract and .

It might be: an ovarian torsion or ruptured . In the case of a torsion, something has caused the ovary to twist, which cuts off its blood flow; ovarian cysts, on the other hand, are quite common and usually unproblematic-unless they rupture or break open. Either condition is serious, says Masterson, who describes the pain for both as “sharp and stabbing, causing you to double over.” You may even experience nausea or vomiting, too.

What to do: Go straight to the ER for medical scans to determine whether a cyst or torsion is causing your severe pain. According to Johns Hopkins Medicine, treatment for ruptured cysts is variable, , but a torsion will pretty much always require surgery to either correct the problem or remove the ovary (if it’s too damaged to be repaired).

It might be: . This condition causes uterine tissue to grow on other organs such as the ovaries and fallopian tubes. It affects up to 10 percent of women, according to the  (ACOG), though women often dismiss their excruciating pain as a “normal” part of their periods. (Again, Masterson notes that any pain not easily controlled by any OTC meds should be considered a sign of a potential problem.)

What to do: If you can, keep a log of your menstrual cycles-noting degrees of pain as well as the dates of your periods and other symptoms like bleeding-and bring it to your doctor. “The more information you can bring about your normal cycles, the better,” Masterson explains, “but if you’re really uncomfortable, don’t wait-just come in and let us figure it out.” Hormone treatments may be effective, she adds, because they can decrease the length of your period and shorten the amount of pain you experience each month.

It might be: a dislodged . Although some mild, initial cramping is normal after implantation, any severe pain or pain lasting more than a few days might indicate a problem with your IUD’s placement.

“Anytime you’re inserting something into the uterus, [it’s possible] it might not be sitting the right way, or could have been dislodged or expelled,” says Masterson.

What to do: Make an appointment with your doctor, who will do a pelvic exam first to see if the IUD strings are visibly coming out of the cervix. If not, an ultrasound will likely be performed. “We want to make sure it’s in the location it’s supposed to be, and hasn’t moved or migrated,” Masterson explains.

It might be: an ectopic pregnancy or miscarriage. According to the , an ectopic pregnancy (which happens in about one in every 50 pregnancies) occurs when the fertilized egg attaches itself somewhere other than the uterus, like the fallopian tube.

Unfortunately, this egg won’t be able to develop and the attached tissue may cause severe bleeding. Both an ectopic pregnancy and a miscarriage (which may occur in ) may cause cramping with an abnormal amount of bleeding. Masterson says one of the key clues here is whether you’re due for your period or not: cramping and bleeding irregularly, outside the usual timing of your cycle, is cause for suspicion.

What to do: First take a pregnancy test, recommends Masterson, if there’s any chance you could be pregnant. If it’s negative, you can rule out a pregnancy-related issue. But if it’s positive, get to a doctor right away-an ectopic pregnancy can’t develop normally and , she adds.