You're not pregnant but your period has changed. Here's why.

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You’re the proud owner of a menstrual cycle, but it’s impossible to be its mechanic. When it does something weird, there’s no manual to guide you or bonnet to peek under. Shouting at your ovaries is not medically proven.

Even for the experts, ‘periods’ + ‘normal’ isn’t straightforward maths.

“Periods vary significantly from woman to woman, and that includes the time between periods, their duration, the amount you bleed, and how many periods you have per year. These all mean that one person’s normal cycle is not necessarily another person’s normal,” explains GP Dr Sam Hay.

When your ‘normal’ suddenly changes, it’s easy to stress. But be reassured to know this: changes in periods are extremely common. On average, 50 out of 1,000 women a year will tell their GP about abnormal uterine bleeding.

“The first question we work out is whether the change is due to pregnancy,” says Dr Sam.

“If it’s not, the truth is that a woman’s regular, clockwork cycle can go out of whack for a few cycles for no significant reason whatsoever. It happens all the time,” he confirms.

“It means that from a medical point of view, if there are only minor changes, it’s useful to watch and wait and see what happens for three cycles.” (Though if symptoms do alarm you, don’t hesitate to see your GP.)

If your cycle still shows unusual patterns after three cycles, make that doctor’s appointment. They will piece together all other symptoms to get a really thorough picture of what’s going on, which could be one of these main causes.

Wondering how much blood is normal to lose during a period? Watch Shelly Horton and Dr Ginny discuss it below. (Post continues after video.)

1. Contraceptives.

“The side effects from taking contraceptives are unpredictable, and if you’re coming off one definitely expect time to adjust. You’re taking away synthetic hormones and relying back on the body’s system, which needs a little time to kick-start again. That can take up to six or 12 cycles.”

2. Ovulation dysfunction.

“Problems with the hormones that influence ovulation can cause irregular bleeding (think: spotting, heavy periods or no periods at all). Two of the most common ones are thyroid disease and polycystic ovaries.”

3. Stress, over-exercising or extreme weight loss.

“When a woman’s weight drops below a set point, the body sees it as being malnourished and turns off the baby-making process – which means women can have irregular periods or lose them altogether. Stress can have a similar effect, as the stress hormones interfere with the reproductive ones.”

4. Structural problems.

“There may be lumps in the lining or wall of the womb.”

5. A bleeding problem.

“If you’re also getting nose bleeds and bruises, there could be an underlying issue with the way your blood clots.”

6. Infection or inflammation.

“Endometriosis or pelvic inflammatory disease – the latter of which is caused when an STD travels into the womb.”

A discussion on your medical history often acts like a sat nav for your GP; giving them a pretty clear path to which one is most likely.

But it’s completely standard for tests to be required.

“One of the first things the doctor may do is confirm that the bleeding is actually coming from the womb – cervical infections could be the culprit, so often an internal examination is required,” Dr Sam explains.

“A blood test is useful to confirm that you’re otherwise well, and possibly an ultrasound to check that the anatomy is looking fine.”

Yes, it’s thorough, but it’s not so scary. Now: exhale!

Dr Sam Hay is a director of a Sydney GP practice and a medical consultant on The Project and Embarrassing Bodies Down Under.