When having your period your uterus fills with blood for the egg to land on. This is what causes the bleeding during your period. The uterus realizes it's an unfertilized egg and let's the blood out.
With Endometriosis little bits of uterine tissue are found all throught your abdomen. When the uterus signals bleeding to start, all those pieces begin to bleed in the abdomen and that's what causes pain. You won't bleed to death but it can be a very painful disease. Ob/Gyn doctors have a variety of ways to help you. Some women report that after their first pregnancy the pain goes away. (I'm not advising this as a solution!)
Check out the website listed above. The Mayo Clinic is a premiere medical institute & their website shows that. You can also google Endometriosis. I hope you feel better soon!
According to what I've read, the pain is caused by trapped endometrial tissues, which have no way to get out of the body. These tissues cause irritation to the surrounding tissues making them develop scars and adhesions. See http://www.mayoclinic.com/health/endometriosis/DS00289 for more information.