Well, Endometriosis is often suspected if you have heavy, very painful periods. I don't know what other women are like with their periods and Endometriosis, but mine are very irregular - I'll bleed for like 10 days, then start it again about 10 days later. I usually bleed 2 weeks in a month. The longer I go w/o treatment I usually bleed non stop (I've bled 6 months before).
This is a good site for more information:
http://www.4woman.gov/faq/endometriosis.cfm
I think it is more likely for women w/ Endo to have regular periods, but bleeding and spotting between periods (thus why I often have multiple periods a month).
But if you are experiencing some of the symptoms (painful periods, bowel movements, etc...) then your dr is doing the right thing by looking to see if it is Endometriosis.