Hi , I'm so sorry you are dealing with this--it sounds super intense! There are lots of issues that can cause back pain, and since yours is worse during your cycle it may be related to endometriosis. Back pain and intestinal pain are unfortunately pretty common with endometriosis (Here are a couple of articles that say more about this connection: https://endometriosis.net/clinical/ibs-diagnosis and https://endometriosis.net/symptoms/back-pain). It doesn't necessarily mean that there's endometriosis in the spine; pain from inside the abdomen can be felt in the back. But it could be from some other cause too, especially since it showed up after pregnancy and a C-section. Endo is just hard and it's not always clear-cut. If you haven't already, you could talk with your doctor about options to investigate the cause so that you can manage it better. It may also be helpful to see an endometriosis specialist, or a GI specialist (they're the ones who deal with intestinal problems).
Thanks for sharing your story here; I'm so glad you've found this community! You are not alone! Sending best wishes, -Audrey (endometriosis.net team)