Sometimes it is just cramps, but pelvic pain can also be a sign of endometriosis, IBS, pelvic inflammatory disease, and even more serious health issues

Any woman who gets menstrual cramps during that time of the month is no stranger to pain. But when it comes to aches below the belt, there's a fine line between sucking it up and ignoring a serious issue.

A recent study from the National Institutes of Health found that one in three women experience chronic, cyclical pelvic pain-aches that lasts more than six months and typically coincide with their menstrual cycle. And there's a whole host of problems that can cause pelvic pain beyond just menstrual cramps.

"The entire pelvis is lined with muscles, bones, ligaments, and fascia and if there are issues within any of those elements then people can experience pelvic pain," says Sallie Sarrel, a New York-based physical therapist who specializes in techniques to relieve pelvic pain. (Ease some of the ache with Yoga Poses to Relieve PMS and Menstrual Cramps.)

What kind of problems are we talking? Our experts say the most common causes of serious pelvic pain include irritable bowel syndrome, which is responsible for pelvic pain in about 35 percent of cases and is usually accompanied with bloating and diarrhea or constipation; pelvic inflammatory disease, which usually manifests as a fever and pain during sex and could cause ovarian cysts, which can burst and cause an ER visit; interstitial cystitis, which typically makes you feel like you have to pee all the time; and endometriosis, which manifests in extremely painful menstrual cramps that usually get worse over time, excessive bleeding during or between menstruation, as well as pain during sex.

So writing any pain off as just really bad cramps is a bad idea. "Some women wait a really long time before being evaluated and think that the pain is nothing, just menstrual cramps," says Erica Dun, M.D., assistant professor of obstetrics, gynecology, and reproductive sciences at Yale School of Medicine. All of these conditions can cause some serious damage-like infertility and even organ damage-if they're ignored. (Read up on some more Top Warning Signs to Ask Your Doctor About.)

Plus, chronic pain causes changes in your brain receptors, so living with untreated pain can actually cause serious psychological effects, says Amy Stein, a New York-based physical therapist and author of Heal Pelvic Pain.

If your pain is serious, your gynecologist can help you figure out what it's from. But how do you know when it's time to see a doctor? According to the experts, these are the four basic warning signs that something more serious is going on.

1. It occurs outside of your period. Pain at anytime can be worrisome, but if it hits when you're not actually on your period, that's a real red flag. (What's going on the other 20-some days? Your Menstrual Cycle Phases-Explained.) Where the pain occurs isn't always a clear indication as to whether the cramps are harmless either. Pain from normal cramps is typically centralized to your lower back and just below your belly button, but it's not always so easy to tell where it's coming from. "The pelvic region is connected to a lot of things," says Isa Herrera, clinical director at the Renew Physical Therapy Center in New York and specialist in pelvic pain. "The pain may not actually be where you think it's at." Keep a pain journal, she suggests, so when you do talk to your gyno, you'll have a detailed record of when and where the pain occurs and how severe it got.

2. Over-the-counter pain relief isn't cutting it. If you're popping ibuprofen to no effect, you probably have a more serious issue on your hands. "When a woman has cramps that impede her ability to participate in any activities like school, work, sports, social interactions, and relationships during or around her period, they're what we call 'killer cramps'-and they're not normal," says Sarrel.

3. It lasts more than six months. According to the American College of Obstetricians and Gynecologists, pelvic pain is considered chronic when it surpasses the six month mark. But Herrera advises you not to wait that long if you think something is going on. "A lot of women suffer in silence. That's what really sucks about pelvic pain," she says. "If it happens even twice, that's a red flag." Herrera advises going to your gyno after the second time-unless the pain is really excruciating, in which case you should see a doctor immediately. (Even non-alarming pain can affect your life, though. Follow these 6 Ways to Stop Your Menstrual Cycle from Ruining Your Workouts.)

4. The pain is so intense you're considering the hospital. Normal cramps really should never be above a four on a scale of one to 10. "If the pain is so severe that you are contemplating going to the emergency room, then go," says Dun. "It is better to be evaluated than to miss something regarding your health."

The bottom line: Know your body. If something feels off or the pain is persistent, get yourself to the gyno pronto.