One of the best sources of omega-3 fatty acids is cold water fatty fish like salmon. Omega-3 fatty acids offer a host of benefits to the body, one being that they act as an anti-inflammatory agent. Data suggests that a diet rich in omega-3 fatty acids reduces prostaglandins and may in turn result in less severe menstrual pain and overall inflammation.
In one study conducted on women with severe menstrual pain (not specifically due to endometriosis), women were either given ibuprofen or 1,000 mg fish oil per day. Results suggest that the fish oil group had less intense pain and the fish oil appears to actually be a better treatment.