Here's the Newsbeat guide to PMT.
What is PMT?
PMT stands for Premenstrual Tension.
It is a mixture of physical and emotional symptoms that some women experience during the days, or sometimes even weeks, leading up to their period.
It also often also known as PMS (Premenstrual Syndrome).
What are the symptoms?
More than 100 symptoms have been associated with PMT.
These include mood swings, anxiety, feeling tired and bloated, cramps and weight gain.
Nearly all women experience some mild changes.
However, the National Association for Premenstrual Syndrome estimates around 1 in 20 have more severe symptoms and these interfere with their quality of life.
They include depression, blackouts and vomiting. The most extreme cases are sometimes diagnosed as having PMDD (Premenstrual Dysphoric Disorder).
The key is that these symptoms appear during the two weeks leading up to a period, get better once menstruation has started, and return every month.
Why do some women get more severe PMT than others?
PMT most commonly affects teenagers and women in their early 20s.
No-one knows the exact cause, but one theory is that women with PMT are particularly sensitive to the varying levels of hormones in the body at certain times of the month.
It's possible that these hormones also interact with the body's mood-controlling chemicals (such as serotonin) in the brain.
What should you do if you think you have PMT?
There are no specific tests that can diagnose PMT but if you suspect you have it, keep a diary of your symptoms for two or three months, then visit your GP.
This will help you and your doctor to see if the symptoms are related to your menstrual cycle.
It's very important to distinguish between PMT and other problems that could be causing similar physical or emotional symptoms, because the treatments could be different.
Is there anything I can do to avoid it?
There are a number of self-help measures you can try.
Some women find taking vitamin B6 or St John's wort helps.
Reducing caffeine, alcohol and sugar around your periods, and exercising could also alleviate the condition.
Painkillers can be used to treat backache, cramps or headaches linked to PMT.
Keep writing down your symptoms each month to help you predict more accurately how you are going to feel.
What if that doesn't work?
If none of this helps, then visit your doctor.
They may prescribe the contraceptive pill, hormone injections or anti-depressants.
In extreme cases some women are given hormone implants or hormone replacement therapy (HRT).
A hysterectomy is the most drastic step and is offered only as a last resort.
~RS~q~RS~~RS~z~RS~55~RS~)