I am sorry you are continuing to have lower back pain.
Have you talked to your doctor about what is happening? Has s/he offered any suggestions or referrals to other specialists?
Endometriosis can cause lower back pain. Was endo definitely diagnosed via pathology following your surgery? If so, possibly all the endometriosis was not removed at the time of your surgery and there is some active endo causing your current issues.
Adhesions often go hand in hand with endo (the local inflammation of the endo can help create adhesions) so you could have some strategically placed adheions causing issues.
Other things that might be a problem include kidney issues. My lower back will start to ache if I have not been drinking enough water and I have other family members with the same problem.
As you have suggested a herniated disk, have you checked into this possibility? Have you had any tests involving your lower back? MRI, CT scan, x-ray?
Are you keeping a symptom diary that you can share with your doctors?
Be persistent and keep working with your doctors to find a solution for you!