Stress and mood swings.
You know those times where you just suddenly get emotional, or maybe one second you’re really sad and the next second you’re just angry. Those are mood swings- and they're totally normal- especially during a period. When you have your period, that triggers hormone levels to rise. Your body will produce more estrogen and progesterone which are hormones that can affect your serotonin levels [1]. Low and/or unstable serotonin levels can lead to mood swings and possibly stress. Although a high amount of stress is bad under all conditions, it can lead to more painful and irregular periods- and may even cause a stop in your cycle [2]. To control the stress levels, you can do a light workout, do some hobbies you enjoy, and focus on positive things.