I was finally diagnosed with endo and uterine fibroids in '94 after about 15 years of pain and very heavy periods. I had a lot of fatigue and very low energy, especially during my periods which would sometimes last for 2 weeks. As the years went on my whole body became quite run down. I believe that are a number of factors that contribute to this: the degree of blood loss, the constant stress that your body is undergoing trying to heal itself, the hormonal imbalances that are part of endo, and the great likelihood of toxic accumulations.
There is substantial research that links endo to other conditions such as candida and chemical residues in the body...particularly dioxin. Dioxin is a byproduct of the bleaching process of paper, including most tampons and sanitary pads. It is also highly toxic.
The Endometriosis Association is a great source of information about the causes and treatments for endo, including conventional and natural/alternative/complementary therapies. You can alleviate a lot of symptoms with diet changes, natural detoxification and cleansing, herbs and supplements, and natural progesterone cream.
If you are advised to go on hormone treatment such as Lupron to induce a temporary chemical menopause (very common treatment for endo), do a lot of research on it and give it considerable thought before you consent. I went through this for six months, and would never do it again! It is extremely hard on your body, and the side-effects are numerous and very uncomfortable. (See my posts down a bit in response to Concerned Mother.)
Good luck with your surgery! ; )