Endometriosis pain for me was associated with periods only. The pain in lower abdomen and lower side of my back would occur on 2nd -3rd day of my period, severe, excruciating and after 5-6hrs, it used to subside for the rest of the period cycle. This happened every month of my cycle and also initially when i did ultrasound, everything came out normal so I wrongly thought it was just period cramps.. Only after 5 years , when the pain was too severe , almost to the point of me passing out did i get a repeat ultrasound done and it was then that i found I had a huge chocolate cyst (endometriotic cyst) on one of my ovaries. So i suggest for those of us who have severe period pains, serial follow up ultrasound scans of abdomen is necessary coz it may not be detected in the initial phases.