It’s true. Although many people have known this for years, the fact that heat can kill period cramps has been scientifically proven recently.
Most pain is caused by blood being temporarily slowed and reduced. The heat actually helps the blood flow to the area of pain which, in turn, alleviates the pain. Heat will actually work exactly like an over-the-counter pain killer for over an hour!
Heat actually works! Scientists actually studied heat and pain as they do pharmaceutical pain killers, and it works in much the same way! As with pharmaceutical pain killers, heat applied to pain helps temporarily alleviate pain, especially in organs and muscles.
Unfortunately, heat does not work as long as pharmaceutical drugs and you are usually stuck in one position for a while as you use the heat. If you have the time, and you want a natural way to kill the pain temporarily, you can definitely apply heat to the area and it will work.
REFERENCE: http://www.dailymail.co.uk/health/article-394086/Hot-water-bottle-really-does-ease-period-pain.html