I would get everything checked out by a DR. You might have a tubal pregnancy, or a fertalized egg making it's way threw your tube in preporation for implantation.
I do not have endometrosis, but I was having back pain, cramps in my right tube, and it was especially noticable and painfull when I would lie on my back. I started to run a very small fever and went to the DR. After an ultrasound they told me I had a cyst, and that it was nothing to worry about. 3 weeks later I took a pregnancy test and sure enough, I was pregnant!
This is my first pregnancy, and as the egg was passing threw my tube it streched me out and made it painful for me. Tubal pregnancy runs in my family, which has made me a litle more causious. From what I understand (I could be wrong) but something like endometrosis could raise your chance of having a tubal pregnancy.
I had pain for about.... 3 to 5 weeks? Also watch for spotting and a small fever.
Hope this helps.