PMS stands for premenstrual syndrome. In short, it is a group of symptoms that is directly connected to the menstrual cycle. The sneaky thing about PMS is that symptoms may start one to two weeks before you actually start bleeding. So it’s very handy if you keep a calendar to mark off when your period (bleeding) starts. This will make it easier to keep up with the start of your symptoms as well.
Researchers feel that a number of reasons may actually cause PMS. The number one reason is the change in hormones. But then, a low level of vitamins and minerals and/or eating large amounts of certain foods (salty foods -- which make you bloated, or alcohol or caffeine -- which may affect your mood) have been pointed out as the possible culprits as well.
https://www.empowher.com/reproductive-system/content/pms-ing-give-me-break