The main reason why heat therapy is so beneficial for pain is that heat naturally increases blood flow to the place of your pain, this increased blood flow relaxes your tissues in that specific area. 

Now in the case of the wicked menstrual cramps, one theory about why it works suggests that heat relaxes the myometrium. The myometrium is the middle layer of your uterine wall, and it is also where the soft muscles are, which contract and cause pain. 

By relaxing the myometrium, your blood vessels that were once compressed are able to relax and allow more blood flow to your uterus. 

What does the research actually have to say about all of this? A study has been done on a group of women, experiencing heavy crams during their period. 

The women used heating pads at a temperature of 104 to 107 degrees Fahrenheit (40-41 degrees Celsius), and the study showed that the heating pads decreased perceived period pain:

• 27% decrease in pain after one hour.
• 43% decrease in pain after two hours.
• 79% decrease in pain after four hours. 

Aside from evaluating a reduction in pain levels, the researchers also viewed changes in blood flow to the uterus. 

Hopefully, more research and studies on this topic will be done in the future under more scientifically ideal conditions.