The period flu isn’t a legit medical term, but it sure does sum up how crummy some people feel during their period.

Flu-like symptoms such as headache, nausea, and even fever are just some of the complaints that have people wondering if they’re getting sick or going crazy during that time of the month.

The good news: You’re not crazy or alone — period flu is definitely a thing, based on anecdotal evidence. And it has nothing to do with the actual flu, so there’s that.

The bad news: It’s still poorly understood and not always acknowledged in the medical community.

Read on to learn more about why you might feel like you have the flu before or during you period and what symptoms warrant a visit to a doctor.

What Are the Symptoms?

The wild ride caused by hormones can vary greatly from one person to another. Some people experience period flu symptoms in the days just before their period that are part of what’s called premenstrual syndrome (PMS). Others feel lousy throughout their period.

The symptoms are pretty varied, too, and can include:

• nausea
• headache
• dizziness
• diarrhea
• constipation
• fatigue
• muscle aches
• cramps
• fever or chills

Why Does It Happen?

Experts aren’t totally sure about what causes this phenomenon, but hormone fluctuations throughout your menstrual cycle are the most likely culprit.

Before your period, prostaglandins, which are hormone-like fatty acids, are produced to help your uterus shed its lining.

Excess prostaglandins make their way into your bloodstream, which can cause a whole bunch of period symptoms, like cramps, period poop, and farts — don’t pretend you don’t know what I’m talking about.

Cyclic changes in your sex hormones, mainly estrogen, can also cause you to feel run down, along with causing your more run-of-the-mill period symptoms, like cramps, breast tenderness, and mood swings.

Chemical changes in your brain, such as fluctuations in serotonin and other chemicals related to mood states, may also trigger some PMS symptoms, according to the Mayo Clinic. These include fatigue, sleep problems, food cravings, and depression.

Could It Mean I’m Pregnant?

Feeling queasy and fatigued as you wait for your period to turn up could set off alarm bells and have you running to the drugstore for a pregnancy test.

Early symptoms of pregnancy and PMS cause many of the same symptoms, like nausea, bloating, fatigue, and breast swelling and tenderness.

But unless your period is late, there’s no link between common period flu symptoms and pregnancy.