I had a second surgery in October 2011 to look for more and none was found. That was my first thought as well.
did they get out all the endometriosis tissue? if it is still there it can still cause pain.
I would recommend magnesium supplements. I thought i had interstitial cystitis for 2 years. Turns out the pain was due to severe magnesium deficiency. Magnesium deficiency increases substance P, a neurotransmitter that acts as a pain messenger. Within one week of taking magnesium supplements, the bladder pain had improved an amazing 50%. By 3 months, all my magnesium deficiency symptoms (had over 50 symptoms) vanished.
I took magnesium chelated with amino acids which is good for absorption. Also look for forms of magnesium such as citrate, gylcinate, or taurate for better absorption as well. Just to note, do not take supplements if you have a kidney problem. Too much magnesium may cause diarrhea so cut back the dosage if this occurs.